Diablo III Pre-Order Nice Price for UK Found
Diablo III (PC) Pre-Order: £27.91 from Tesco Entertainment using the discount code GAMES5
This includes delivery, and "goods are despatched to arrive on the day of release."
I've preordered my copy here today as it's the best price I could find. It's also £17.08 cheaper than pre-ordering a digital copy direct from Battle.net, but then that pretty much guarantees you can start day 1 rather than be dependant on your postal service/other factors.
If you find a better deal, please post it and let others know!