It is generally possible under English law to assign rights under a contract, just not liabilities. It's also possible for the mortgage contract itself to conta

It is generally possible under English law to assign rights under a contract, just not liabilities. It's also possible for the mortgage contract itself to contain a provision permitting transfer by the bank. Mortgage-backed securities (which generally involve the selling of mortgages to an SPV) are certainly a thing in the UK. For retail mortgages there are presumably regulatory considerations as well, eg, the mortgage probably has to continue to be serviced by a regulated entity. It's not uncommon for the original lender to sell the rights to the mortgage but continue to service it.