It's very much the same as road bikes, in other words manufacturers source a frame, then equip it at various price points with varying (and often predictable) major components, then finish it off with little bits of flair in the finishing kit. SO, you'll find most bikes at the £300 price point have much the same drivetrain, forks, (brakes often differ) and alternate between budget main brand wheels/hubs and the occasional own brand masterpiece.
For every step change you'll be finding another £100-£150, so the next budget point will be £500, then £650, then £800 then £1000. If you think as a rule of thumb that the £300 bikes will have Altus drivetrain, Alivio on 500s, Deore on 650, SLX on 850 and (some) XT on 1000, and the forks will increase in spec too. So expect say RockShox Dart on £500, Rebas or fox floats on 1000 bikes and you start to see the picture.
The difference in frames will be subtle. Most will be using 6061 Alu progressing to 7005, with butting, double-butting and hydroforming appearing at the incremental points. Geometry will differ, with Euro-centric bikes being racier with steep head tubes for fire road speed, to slacker head angles and longer travel forks on Brit -centric trail bashers, so what you intend to ride will determine your choice. You sound like you would be looking for a 100mm travel XC bike, but don't ignore European "trekking" bikes.
The fact that they all use much the same kit will help you to narrow the choices down to simple stuff like colour, love at first sight and other niceties. Don't get hung up on finishing kit. most OEM stuff is pretty naff anyway, and you'll soon want to treat yourself to a nice pair of lock-on grips, or a nicer saddle, a wider pair of bars or a shorter stem, so don't base your choice on them necessarily!
Some brands tend to offer half a grade better kit than others (Cube spring to mind) where others appear, initially to be a bit dearer than others (Specialized a good example) at the price point.
Fork lockout is not as important as some would have you think. I rarely lock mine, and they are super plush. I simply alter my pedalling technique to keep the weight off the front. The only exception is steep tarmac climbs. Far more important is a half-decent set of forks with damping adjustment, so that your bike doesn't ping you off on the next rut.
Get discs if you can afford them. Full stop!
