Welcome aboard
As others have already mentioned, a GBB is indeed more realistic in terms of how it works, which is obviously appealing, but I'd agree that it is probably not a great choice for your one and only first gun. Gas blowbacks take a bit more maintenance and can be temperamental, especially in this cold weather, which is not a good thing when you are new to stuff, not to mention the fact that they have small capacity magazines and require gas (tenner for a can of it), which means you need to buy probably at least three or four spare mags. Spare mags for gas blowback rifles tend to cost 30-40 quid a pop (unless you buy them from abroad, when they are about 20 quid), so that'd be another 100 quid or so you'd be looking at to get something you could skirmish with easily, and of course it would also mean you'd need some kind of suitable tactical vest in order to carry the mags as well (cos GBB mags are heavy), so that's at least another 30 quid again.
Yes it is fun and realistic to yell 'I'm out, reloading!' in mid skirmish, but it can get old soon in comparison to simply rocking a 300 round mag on an aeg, with nothing more to do than occasionally twirl the little wheel on the bottom of the mag to keep things feeding. Going with 30 round capacity gas blowback mags is only really enjoyable if you are at a military sim airsoft skirmish, where you know everyone else has the same thing to deal with, at a more typical skirmish, it'll just put you at a disadvantage. Besides which, it is possible to get 30 round aeg mags too, which are cheap, if you fancy trying a bit more realism, but I'm betting you'll be popping that high cap mag in before long when you are pinned down by someone with a high cap aeg mag!
Far better to have something which just works without any worries straight out of the box, which is what electric ones will do after you've simply charged the battery up and slung it in the thing. Most of them come with a high capacity mag which can take 300-500 rounds (and between 2000-4000 rounds if you stick a drum magazine on them, which you can find for 20 quid), which is enough to last an entire skirmish if you don't spray automatic fire all over the place, but even if you do, high cap mags for AEGs are usually only 5-10 quid a pop, so the additional mag cost is negligible, they don't weigh much either, so they'll go in a pocket of some combat pants no bother and are almost disposable at that kind of money.
You'll probably get a lot of recommendations for something like the Combat Machine M4 from G&G, which is an excellent choice for airsoft. I've got one myself and even though I have a lot fancier guns, that one is a firm favourite of mine, because it is reliable and fun to use. You can find 'starter deals' which include the G&G combat machine M4 and a few other necessities, so it's a good way to get started for not a lot of money. Here's a few for example:
http://airsoftzone.co.uk/airsoft-zone-deals
Some other decent starter guns to consider if you want to get a gun alone and get the protective gear separately (watch out for guns that don't come with a battery or a charger by the way, which some places do to keep the price down, which is a bit misleading, although the ones I've linked to usually do come with that stuff):
117 quid:
http://www.zerooneairsoft.com/product_info.php?products_id=5645
139 quid:
https://www.youtube.com/watch?v=9PMdp_l-Hv0#t=403
104 quid:
http://www.onlybbguns.co.uk/g5-g5.html?filter_name=Galaxy%20G5#.VNeDli5JVhM
140 quid:
http://www.patrolbase.co.uk/two-tone-airsoft/umarex-g36c-two-tone.htm#.VNeEHi5JVhM
270 quid:
http://extremebbguns.co.uk/product/ares-twotone-am-013-6mm-electric-airsoft-honey-badger/
Note that some places (principally websites with BB in the name, which come in for a bit of flak on this forum) tend to sell mostly two-toned guns (this is worth knowing, because other outlets can sometimes charge you twenty quid to spray the thing two tone), which would be the only thing you could legally buy unless you could prove you were a regular skirmisher, which people invariably do by applying for a United Kingdom Airsoft Retailers Association (UKARA) registration number. A UKARA number allows a website to check up on your airsoft credentials when you are ordering stuff online, so they know you are buying it for a legitimate reason. You can get a UKARA number by having attended three skirmishes at a professional skirmishing site (ask them about it when you go). In the interim, it is often smarter to simply hire a gun from a skirmish site, then apply for a UKARA. That would enable you to order an airsoft gun from a company abroad without any hassle (since the border agency do look for UKARA numbers on packages, and can confiscate things if they think you are 'importing' an airsoft rifle without a legitimate reason). Airsoft guns are vastly cheaper from such foreign websites, so this is a good idea if you are prepared to wait before buying..
The other advantage of doing that, is you can dip your toe into the water a bit with skirmishing using a hire gun (which will most likely be a Jing Gong Heckler and Koch G36 assault rifle, which is another excellent first weapon incidentally), and get a feel for the kind of weapon you would like to buy, plus you get to see what everyone else is using too, what sights they favour on their guns, whether they also carry a pistol etc. Doing that is very useful, because if you ask nicely, most everyone at a skirmish will let you have a try out of their weapon (we are a friendly bunch with a shared interest), and that is very helpful, because you do sometimes find that something you like the look of actually does not sit well in your shoulder, or fit your hands very well, or you may not like the position of the fire selector switch or whatever when you actually hold the thing. For example, the fire selector switch is a bit tricky to reach on an AK47 if you have small hands, the fire selector switch on an M4 can easily get knocked onto a different setting when it rubs on your chest if you have it on a sling, the H&K MP5K can be tricky to point accurately if it doesn't have a shoulder stock, but it has an ambidextrous fire selector switch, the G36 has an ambidextrous fire selector switch and cocking lever, and a folding stock, so it suits left handed people and is good for CQB and long ranged fighting, and so on. There is no better way to determine if these features will suit you, than to actually hold the things.
If on the other hand, you cannot wait to get something (yes that happens a lot when people are excitied and keen to get into it), and do wish to get a two toned rifle right away, then do your best to stay away from the crap stuff, by using this forum. Ask people on here about weapons you are considering purchasing. Some stuff can be had for not a lot of money from various sites such as: bbguns4less, onlybbguns, justbbguns and bbguns365, geniestuff, and that is a tempting proposition when you are new to things and not sure what is good or bad. Be aware that some of those sites are known for selling stuff which is not really up to the task of skirmishing against others at professional airsoft skirmish sites, yet claiming that they are (they tend to claim that everything they sell is awesome). Not everything they sell is crap, and they can quite often be the cheapest places to acquire a particular airsoft gun, but do not take the reviews on those sites at face value, check other sources, look for reviews on youtube and ask about things on forums before you hit that 'add to shopping cart' button, because if you buy something which sucks, you'll end up having to buy something else not long after. Many of us have been caught out that way on some occasion in the past, but you don't have to join that club!
Above all, take your time before committing to spending money, the internet is your friend in this respect, and you will find a bit of research will go a long way and save you money in the long run. Oh, and don't forget to get some decent eye and lower face protection, and a decent scarf for your neck, do not go cheap on that, you only get one par of eyes, a BB can easily smash a tooth out, and a BB hit in the neck is bloody painful and will draw blood, so cover yourself up.