There are all sorts of pitfalls and the stamp duty will have to be paid on
the open market value. In order to minimise the costs and make the best use
of the tax exemptions that are available you must use a solicitor. His fees
will far outweigh the savings you might imagine you could make by DIY ing
it. Should not cost more than a few hundred pounds. They can also deal with
the conveyance that will be needed at the same time. The new building
society will probably insist on the searches being done anyway. You will be
VERY ill advised if you act on such a transaction without professional help.
IANAL but have been involved in a similar transaction.