Hi ABGD, having built an RV-4 (too) many moons ago, I'd say order a tail kit from Vans. I'm sure you'll find someone in the UK willing to put it together if you don't want to do it yourself. It should not take too long, but be prepared to spend some time on the necessary jigs.
As for buying already built elements, adapting them to your aircraft will present some challenge if already drilled for the horizontal/vertical stabs... if the holes are not exactly aligned one could go to bigger size bolts, using caution to respect edge material limitations. If the tail feathers are not drilled, no problem, but careful match drilling required.
Good luck!