£350 takes you out of the ICS/G&G £150-£200 leagues. I'd say just pick a gun type you like the look of and get the Tokyo Marui version of it, they're the godfathers of modern airsoft guns.
There's lots of exotic options for that cash, but for a first gun you can't beat a bit o Marui, as it'll be a stable foundation to build your collection from, and will probably never break unless you give it a helping drop/kick. It's a gun you'd not really need to upgrade ever, perhaps buy it with a stronger spring installed or treat yourself to a higher quality barrel, but that's about it.
If you're picking a gun type you've not held before i'd recommend going to an airsoft store to give it a feel, even if you buy from elsewhere.