If you're dealing with a big dealership and not some fly-by-night back-street place, I wouldn't be too concerned. Registrations can be misplaced, and it sometimes takes DVLA several weeks to issue a replacement.
The British registration is not really equivalent to a certificate of title, since it doesn't show ownership in any way. As DVLA points out on the forms, the V5 shows the "registered keeper," who is not necessarily the legal owner. There is no actual title or other official document in the U.K. to show ownership of a vehicle.