The Blue Cross are very good, in my opinion better than the RSPCA, they really research their dogs and give a lot of backup when they rehome a dog (all their dogs are insured when they leave the centre, for six weeks). I find the RSPCA vary a lot from centre to centre, some are good but some are bad.
Why not get in touch with a local rescue centre, many often need volunteers to help walk the dogs etc. This way you can familiarise yourself with the dogs up for re-homing and gain experience of looking after dogs. Sometimes they also need foster homes for dogs (which sometimes turn into forever homes!).
If you do get a rescue, make sure that the place you get them from checks you out and is prepared to give follow up advice. Because this is your first dog you do not want to get one that has alot of problems.
A good web site for dog training/behaviour and general information is
http://www.takingthelead.co.uk they have lots of info. on dogs and dog breeds. They also have a good forum. One of the people who runs it is Gill, she works for the Blue Cross with their dogs and she is a very experienced behaviourist.
Good luck in your search!