Below are some links to recommendations. As much as people slag off 'BB gun' sites (especially on these forums, and not always undeservedly so), I've actually had pretty good service from them whenever I've bought things off them. However, that's because you generally have to know what you are looking for in order to avoid the turkeys, and those sites often 'talk up' guns which are not that great. But if you do know what to look for, they can offer some good deals. It is worth noting that if you do not have a UKARA registration, you'll probably be looking at having to buy a two-toned M4, which is no bad thing really, since you can either keep it like that, or if it bothers you, get some matt black spray paint and just spray it the right colour (note that you need a suitable reason for doing this if you want to be within the boundaries of the law, i.e. if you genuinely want it for airsoft, and do so regularly). This is where buying from a 'BB gun' site makes sense, because those sites tend to mostly sell two-toned guns, so you won't be charged for them painting a black weapon in two tone colours, because it will already be two toned.
Here's a few links to some decent two toners from a BB gun site, all of which are pretty affordable. And before anyone chips in and says those sites are crap and don't buy from them, I will say that the first link in the list below, is to the D-Boys M4 SIR, at a very reasonable 179 quid, and I in fact bought that exact gun, from that exact site, in that exact same two tone blue and black. I simply sprayed the fore grip and the stock black with a cheap ass aerosol can of matt black paint (the paint literally cost me 1 pound from a cheap shop), and it looks great. The D-Boys M4 SIR is a really excellent rifle, it is in fact my main skirmish weapon for both CQB and outdoors, and I've never had any problems with it at all (and I've got loads of different rifles and SMGs, so it's not as if I am forced to use that one), I use it because it is a truly excellent weapon. Apart from painting it, the only thing I've added to it, is a pressure activated TAC light on the right fore grip rail, with the pressure switch gaffer taped to the front fore grip. I don't even use any optics on the thing, because the iron sights are excellent and I can hit people at long range using those quite easily:
http://www.onlybbguns.co.uk/dboy33-d-boy-m4-sir-airsoft-gun-aeg-metal.html?filter_name=D-Boys%20M4&page=4#.VH7tbNxd1RU
http://www.onlybbguns.co.uk/cm008-cyma-m4-sir-cqb-airsoft-gun-metal.html?filter_name=D-Boys%20M4#.VH7sKtxd1RU
http://www.onlybbguns.co.uk/061-jg-m4-gas-blow-back-assault-rifle-gbbr.html?filter_name=D-Boys%20M4#.VH7sbtxd1RU
http://www.onlybbguns.co.uk/032-agm-m4a1-socom-aeg.html?filter_name=D-Boys%20M4#.VH7sidxd1RU
http://www.onlybbguns.co.uk/062-jg-m4-gas-blow-back-assault-rifle-with-ris-rail-gbbr.html?filter_name=D-Boys%20M4#.VH7sxdxd1RU
http://www.onlybbguns.co.uk/031-agm-mp031-m4-rifle-full-metal.html?filter_name=D-Boys%20M4#.VH7s7Nxd1RU
Note that a couple of those recommendations are for Gas Blowback M4s. Those won't work well in the current low temperatures, and you would need two or three spare mags for them, which would probably add another 50-70 quid to the cost, so unless you really must have a GBB weapon, I'd say stick to the electric ones with the high capacity mags. The D-Boys M4 SIR is therefore the one I'd recommend to you, since it is electric and has a 300-round mag with it, which means you shouldn't even need to reload it on a skirmish, just roll the winder on the mag every fifty shots or so, and if you use it on semi-auto and double-tap shots, it might even last you the whole day. The battery will certainly last all day if you do that, I know that for a fact, because I did exactly that with mine the Sunday before last at Trojan's woodland site out near Macclesfield, which also demonstrated how tough that gun is, because I was crawling and rolling about in mud and crap all day with the thing and it never missed a beat.