Well, you mention indoors, fair enough £300 is a good budget for a CQB kinda gun as they are usually cheaper because less materials have to be used to make them.
My all time favourite is the KWA MP7 its gas I know but the delivery system on that gun is amazing and I easily fire off 2 full mags on 1 pop of gas. Means less gas used, means more money saved up. Or you can just go propane and an adapter to feed the silicone in.
Another amazing gun that Ive actually been looking at getting is the ICS CXP Concept rifle. You can get it either in metal or plastic. If you arent clumsy and you know you wont break it quickly Id go for the plastic as its around 120 quid cheaper. It foes for I believe 170-190 and the metal goes for 280-290.
Its a beautiful looking M4 platform and yes there are a ton of M4s around but this one actually catches my eye. I do believe you might have to detune it to 300-330 if you wanna play indoors because someone has told me it fires around 400 which is a bit much for a CQB gun. Dunno what ICS was thinking.
I cant remember who made it but a lot of people recommend P90s. They are tiny they are bullpup which means you get a longer barrel as everything is located right by your shoulder. Meaning more space for the barrel.
The TM AK74u is beautiful, fires 300 FPS, has a foldable stock and it is tiny in itself. All those features make it a great CQB gun and its TM so you are getting what youve paid for. Never heard about anyone having problems with these other then after some time the gears wear off but thats to be expected from a gun taht is mostly being used in CQB as you fire a little more then if you were playing in woodland.
Definitely get something on which you can mount a torch. Blinding the enemy is useful and you can see around dark places.
If you end up going for a gun which has no torch mounting options you can always get a helmet with rails and mount your torch there. I do it because its always shining where I look and not where the gun is pointing.
I have the MP7 and although the 40 rd mags which cost £40 each (mental) is a bit disadvantageous I love the feel of the thing, it is so loud everyone runs away from me when they hear me coming muahahaha! >
My second choice would probably be either the ICS CXP to try it out or the TM ak74u because its reliable and it already fires at 300 and for CQB you dont need a tightbore or anything. You just need loads and loads of bbs coming out of your barrel