I hate PC shopping. some people like it. I can't fathom. IMO, getting out of the PC market and into your games is the reward for slogging through that shit.

if it wasn't a problem, the Steam machine initiative wouldn't exist.

I know nothing of the UK market for PCs and it's more than just a case of currency conversion. so because of this, I have no idea where your $350 GBP budget puts you, sorry.

some things I'd share

in 2016 we move from 28nm process node which we've been using for years to 16 and 14nm processes which are double the density. which means, GPUS, get their next big bump. which means, if he buys into anything current, right now, he might regret it when it can't play shit. now, I think this is why NX is launching sooner than later and I think it will be fairly powerful because nvidia and AMD want to push a short console cycle by using Nintendo as a lever, so they got a nice rate on whatever chip is going in it.

but all this means that if he buys an expensive GPU there is no telling how long it will be good for. people say "my 970 will last me a long time" but with nvidia there is 0 guarantee of any kind of software support. so even if the thing can run the game, if it relies on driver-side optimizations, it might still run like crap if those optimizations haven't been done. VULCAN and the new opengl initiatives do mitigate that concern somewhat. but we have to see how that shakes out. the only thing that can be depended on to counteract any of this is that with the economy the way it is, people don't like to upgrade and like to get more of their cards. and developers want to sell their games to more than just a tiny bunch of folks with substantively marked up hardware.

prebuilts often have weak PSUs. this is partly because they don't need a lot of juice, and partly because more mainstream upgrades like ram and storage drives don't consume a ton of extra energy. it almost means that you might need to buy a PSU later if your PSU isn't sufficient for a graphics card upgrade. the wattage of PSU is something you need to take into account if you're looking at upgradability. in addition, prebuilts often have slimmer, custom cases. these cases can interfere with adding a normal sized PSU and you may be forced to buy a much more expensive small form factor PSU if you can find one that will fit at all.

buying an FX based prebuilt, while you may get a good price, comes with some heavy disadvantages. poor single thread performance (still important for games though vulcan and new opengl could change that), higher power consumption, older motherboard platform lacking in modern i/o like USB3, m2, sata express, and USB3.1.

generally speaking, the best bang for your buck is buying a prebuilt and adding in a low-TDP GPU like 750ti or 260X/370. however, other cards than the 750ti need a 6pin or 8pin lead, which a prebuilt PSU may not have. again, this could all change when the new cards drop and it could end up being that you get cheap bins of these new cards (because yields on new stuff are shitty. they bake the stuff like cookies and some come out good and some do not) that are energy efficient enough not to need 6pin or 8pin lead.

pay attention to whether or not your PC comes with win10 or whether you're in the upgrade window because Microsoft may yet keep dx12 exclusive to win10 if developers do not support vulcan strongly enough.

not to burst your bubble, this is after all just one person's opinion, but in general, buying a PC for gaming, again, in my opinion, is a shitty experience of mark up acceptance or heavy compromising. maybe steam machines will change this a bit.