I don't see how you couldn't have a full metal receiver if you're spending £300+
It's not so easy to say because there are so many airsoft AR-15 style guns.
I'd recommend one of these (http://uk.redwolfairsoft.com/redwolf/airsoft/AEG_AEP_Magpul_Magpul_PTS_RM4_CQB_ERG_Black.htm / http://www.landwarriorairsoft.com/airsoft-weapons-c38/electric-rifles-c45/pts-pts-rm4-cqb-erg-p3303) they're within your budget but there's only the CQB version in stock.
They are awesome from what I have heard, and plus you get the recoil / vibration.Yes it says CQB and is shorter but in airsoft, it's actually the barrel quality that matters more than the length and bore. So you will probably have the same performance as the Scout version anyway.
Plus I know there's one or two guys on here who have one - I was going to buy one too but decided I'm going to buy an AUG then get a GBBR M4.
If 416s are your thing:
http://www.landwarriorairsoft.com/airsoft-weapons-c38/electric-rifles-c45/vfc-umarex-hk-416-p3178
http://www.landwarriorairsoft.com/airsoft-weapons-c38/electric-rifles-c45/we-we-416-p3358
Have a look at G&P too, they start from around £250