If you have time and money, then go out get your UKARA defence. It's not hard to get one.
If the reason why you don't have it is because you're under 18 then I would recommend camo tape too.
Legally I would say no, you're not allowed to do it. It doesn't matter how or where you're going to buy the parts - it's still going to be a RIF. That said, if you can use camo tape (you must take it off after a skirmish day so the gun is two-tone again) then in theory it would be okay if you bought black parts, but only put them on at the skirmish site, and then replaced them with the bright coloured parts after. It would take no time at all to switch your handguard and stock and there you go.
Best thing to do about that is ask your local site, and remember always keep your gun in a bag when transporting it etc.