Particularly if you are intending to get premium tyres, you should give seious consideration to dropping to a smaller size of wheel first. The 17" sport wheels look superb of course, but they are heavy, uncomfortable and notoriously prone to buckling.
The standard 16" SE six-spoke wheels are very durable, as of course are the 15" pepperpots; both will give a more comfortable ride than what you have now.
I don't have a particularly good source of tyres in Norwich I'm afraid. I've shopped online for two particular tyre types, and then gone with the cheapest local option. Formula One Autocentre on Barn Road have done a decent job of supplying and fitting my tyres, but I've declined any service beyond the basic fitting, and definitely not allowed them to adjust my tracking despite their claim that it was pretty much essential!
JSF 4x4 on Spar Road have the full Hunter system and it's worth booking yourself in there if you need to have your wheels correctly aligned.