Verified answer

You need to use the services provided by the UK Land Registry.

There are various types of searches you can make and you need to pay a fee for them.

See link below.

EDIT: Regarding your additional info: I believe you are mistaken.

Land Registry records are publicly available (except for the Lease Register) and every time a property sale is notified to the LR details of the sale price are also included. You complete an LR1 form for a search of the Title Register (£20 for a single search, or a discount on multiple searches with up to twenty searches on a single LR1). To quote the LR site this search gives you "...a description of the property, its tenure, the name and address of the owners, purchase price, details of mortgages and other charges, covenants etc..."

You can even combine a title search with one for all registered old deeds using form LR6, but older records do not always show sale prices.

The search I most commonly use is LR7 (Registered Owner name and address) which can be done online and often in under an hour.

See additional link below