I've been struggling a little recently too, i keep forgetting to book preorders then have to go crazy trying to hunt the games down - i think i secretly enjoy this part
Anyway, i've been having some good success using Argos.co.uk, you can check the stock for any local Argos store. You can reserve the item if it's listed and they'll give you 48 hours to go collect it.
I checked this on and off all day on the GTA4 release date and got lucky around 1pm, reserved the game and was playing it an hour later.