I'll need to be the voice of reason here and suggest that - due to the climate in the UK (and not withstanding your smaller budget) - you should reconsider your choice of a GBBR as your first gun. They're typically more expensive than an AEG and all have certain nuances to do with keeping mags warm, using more expensive gasses in the winter, maintaining magazines etc. that aren't newbie friendly. I'd also avoid doing it on a budget as the magazines alone will run you about £150 quid regardless of which you buy. Cannot understate just how poorly many GBBRs will perform in the late autumn up until early spring. GBBRs are also not as cheap to run - green gas is quite expensive, and the higher powered stuff is more expensive still. However, you might recoup some of that cost in using less BBs.
If not, TM MWS comes closest to being a consistent performer through all the UK weather, but personally I'd go for anything GHK. Neither are cheap, and for that reason alone I'd avoid it. Budget GBBRs are also usually a bit crap (exception being some of the WE stuff), but second hand you can find some good deals. Again though, the magazines retain their value and will still cost about £100 for 4-5 or so.
Buy an AEG as a starting point and then you always have something to fall back on later down the line. G&G CM16s or some of the ICS stuff is recommended fairly often. Also, your profile says you already have a G&G GBBR?
As with all of these 'wot gun' threads, this is a bit too much of an open question: For the best results, do your own research first then ask questions. It sounds like you've not done much at all.