You value service yet went to Arnold Clark?
Have you thought about a Euro Import DMW? www.motorpoint.co.uk have some good prices just now, although you'd need to sell your own car privately as their trade ins are rubbish. I got good service from motorpoint in actually buying the car, but after that you have to rely on either main dealers or subaru/jap specialists for servicing etc. as motorpoint are sales only really. You'd get a new 2.5 STI import for around your budget.
In terms of STI over the WRX. Tough choice really. Test them all and see what you think. I've got a WRX and I'm really happy with it, but I couldn't have afforded a new STI at the time so I was limited.
Whatever you do end up with you'll love. But you'll never have enough POWER!