Interesting set of requirements from a newbie! Certainly sounds like you know what you want. What has driven you to this budget/spec?
I'd say have a look at an ICS L85 http://www.patrolbase.co.uk/airsoft-bulpup-guns/ics-l85-a2.htm#.VDZeyvldWSo
Not sure on weight but it's a bullpup, takes pretty standard internal parts, uses STANAG (M4 style) magazines and it's £100 under budget.
The TAR21s all have issues to some degree or another and the UAR is like hens teeth and apparently (I've never held one so I can't say for sure) feels very plasticky and cheap.