Ocean Safety in Saxon Wharf would be worth a call, as they supply and service many of the commerical rafts. They also service many of the rafts people take into the local chandlers for a service.
Suffolk Sailing also have some good prices from time to time.
Also worth thinking about what goes in to the life rafe for long term costs. If the extra flares for SOLAS B are caried in the raft then you will need additional ones on board to satisfy the surveyor, (you can't open the raft just to get the flares) same for termal protective aids (TPAs). If these items are carried in a grab bag instead of inside the liferaft you will only need one set in addition to the basic ones in any raft, which in 3 years time is one set to replace not two and one set of TPAs to carry not two. Lower initial and ongoing costs in addition to storage, which always seems limited on a rib.
Gary at Messenger Marine services my liferaft annually, would be worth also having a chat to him and whoever you intend to do the survey before you buy.