In reply to woolsack:
I know your OP is about online resources of information but I thought it worth mentioning that Boots often has some sort of offer (used to be 3 for 2, now the slightly less enticing "buy one, get the second half price") on film, and they tend to have Ilford HP5+ in stock. Worth knowing if buying in small quantities as AG and so on have a fixed minimum postage price.
Some places sell via Amazon too.