google it theres loads
also worth noting that its very tyre dependant
if you have a tyre in mind id google the brand and size you want.
cam skill do some very good deals a lot of the time.
 
camskills always worked out the cheapest for me and have always been very reliable as well.
 
I use a local independent, get prices off t'internet , he'll normally match or beat them. Plus I know the guys that do the fitting and they look after your alloys. Oh and the coffee's cheap.... :D
 
Mytyres used to be cheap but I haven't found them to be very cheap for a few years now. Camskill are normally the cheapest but they are quite often out of stock.
 
I've not used mytyres since I had a problem returning unwanted tyres.

Sent them back as agreed but because they aren't based in the UK I had to fight tooth and nail to get my money refunded, had to go small claims to eventually get my money back.
 
Independants as already mentioned will generally match an online price and remember you will have to pay to get online tyres fitted as well.