If you could type in a registration number to get the details of the registered keeper of a vehicle, it would be a gift to weirdos and stalkers. (The DVLA will sometimes provide such details but only to people who can provide good reasons, and with a record of the check being kept so that any people abusing the system can be appropriately dealt with).
Any company seeking to bypass the DVLA's system (by providing the type of service you're seeking) would be prosecuted under the terms of the Data Protection Act.
Anyway, there's no such thing as a register of vehicle owners. The only name and address recorded by DVLA is that of the registered keeper. For example, a company car driver might be the registered keeper of the vehicle (so that any speeding tickets are sent to him), even though the vehicle is owned by his employer (or, more likely, by a leasing company)..
To check that the person selling the car is the registered keeper (even though that tells you nothing about the ownership of the car), simply make sure you see the V5C document (and only buy at the address shown on that document).
Chris