For that money, I'd say any of the WE gas rifles. All other brands are going to cost more with the possible exception of the KJW or AGM gas M4s, but with those you'll struggle to find any spare parts if they break - the AGM is famous for breaking and although the KJW is regarded as one of the best out of the box gas blowback rifles, spares for it are more or less non-existant. You'll probably struggle to find the KJW and AGM M4s to even buy one in the first place. Same goes for almost anything aside from WE and GHK, come to think of it. The G&P WOC series seem to be about as rare as bird's teeth and the Inokatsu gas rifles tend to be too expensive to be worth the effort.
Though even with the WE rifles, you're going to need a lot of spare mags, as well as money set aside for replacement parts, since moving parts are all going to wear out eventually, and it tends to be sooner rather than later with the pot metal WE tend to use, especially in their trigger mechanisms.
Or you could look at smaller SMGs or PDWs like the KWA or TM gas MP7, or the KWA MP9. They're tried and tested and perform very well, I was looking at getting one as a sniper back up/CQB primary.
You're unlikely to get a skirmishable gas gun that isn't a pistol, for around £300 in fairness, I've even spent that on my pistol... Gun, mags, holster, new barrel 'cos my last one exploded...
Most gas guns that aren't pistols tend to be £250ish on their own, with magazines costing anything from £30 to £70, and only holding between 20 and 50 shots. Then there's gas to think about, lubricants to keep the magazines in check etc etc.
If I were going to make the move into gas blowback rifle skirmishing, I'd set myself a budget of around £1k, get a fully decked out WE rifle, fitted with all the RA Tech upgrade parts available, then spend the rest on magazines.
But! If you just want something as a back garden plinker, then any of the WE rifles will do fine, and you ought to get by on just the mag it comes with, putting most of them more or less bang on the money at £300.
Take a look on Milspec Solution's website and give Iain a ring, he'll help sort you out.