Jambo Jez,
I’ve moved this question to the Mortgage questions forum. If you look around in this forum you will find lots of questions from people in a similar situation.
The lender can chase you back in the UK, but I doubt they will chase you in Kenya. Where is your declared residency on the mortgage deed?
Are lenders chasing people back in the UK? I’ve asked this question many times, but never come across anyone to whom it has actually happened. The banks have to weigh up size of the mortgage and the cost of pursuing the borrower outside of Spain.
In your case, if the lender thinks they gave to chase you in Kenya, and don’t know about assets in the UK, they will be more likely to accept a dación en pago (handing back the keys).