Try www.ror.org.uk they have the details of five of the most reputable re-homing centres. Or, you can contact the trainers direct at your local racing stables.
Festive Firewell went to some rehoming centres recently and has taken one on, it would be worth sending her a pm in case she misses this thread. If you do a search you will find her report on how the visits went and updates on the horse, I think it is her mums.