i'd be looking at the invidia route for an off the shelf job, but its over your budget,
If you want quality workmaship and under £500 then i would be looking at a custom built exhaust from someone like longlife.
My XKR exhaust system was built by them very good workmanship. I used the guys near ipswich called Demand Engineering, i'm back there next week for some updates to the system, they are changing the tips for me to fit my new bumper cutouts. they are also going to be doing another forum members car soon as well and from what i remember the quote was on the right side of £500. there are other longlife places in the UK but i know this particular branch are very good.