+1 to play before you buy, except eye protection. Most sites will have a full face mask with a clear polycarbonate eye shield for customers who rent, but these things fog up something bad. My local site also insist that under 18's wear full face protection; you had best check that with the site/s you intend to try out.
I would suggest that you buy some mesh eyepro and a separate 1/2 face mask like these
here and
here. You may read/hear scare stories about Chinese products, or ACM (All China Made), but I have and continue to use them myself, even after testing some to destruction; obviously it's up to you to decide whether I would deliberately advise you to do something dangerous just for the lols or not; something which you can verify for yourself however is that there has never, not even once, nowhere, under no circumstances, been an incident where Chinese eyepro has failed during an airsoft skirmish leading to someone being blinded or even injured seriously enough for it to be reported.
If money is an issue, for your first games you could go for
this. If airsoft turns out not to be for you, you'll not have wasted much money.
+1 for not buying from Just BB Guns - if you search this site you'll find plenty of first hand reports of them being wankers.
I'm in no position to advise on AR15 variants as I hate 'em, but, if you already have some STANAG mags, you're pretty much committed, unless you go for an SA80 variant, which would probably be better, but also more expensive.
Camouflage wise, DPM is designed for Temperate Northern Europe, which includes Britain, and jungles - it's cheap, easy to get in a wide variety of sizes, and easy to get on loads of different types of clothing, from lightweight summer gear to full-on Arctic. I agree with Ed about Flecktarn being the best for actual use as camo in the widest variety of British environments, but it's not as easy to get in all the possible sizes without spending, on average, something like twice as much as for DPM for less common items, but for a simple trousers, shirt, and cap or helmet cover, if you're a common size you should be able to get what you need for not much more than DPM.
In the UK these days a lot of people are wearing Multi Terrain Pattern, Multi Cam, or a mixture of the two - not surprising since that's currently the prevalent western military style. There is an ongoing debate about how suited to British environments it/they is/are, but there is no question that it was designed primarily with more arid climates in mind and temperate Europe as a secondary concern.
A lot of it will depend on whether you want to fit in, or wear something different to everyone else, and whether you want to wear an authentic uniform or just go with what you like the look of. Remember that you don't have to stick to one type of camouflage; even some military units wear mixtures such as MC & Coyote Brown, MTP & Olive Drab, etc.
If you do want to go for something less common, you might want to look at Italian Vegetato, or Russian Flecktarn, Cadpat, Danish gear, or whatever lol!