I’m afraid there are no simple answers to this extremely common problem for expat (British) Spanish property owners.
If negotiations with your Spanish lender fail, can you still afford to keep up the payments? If the answer is No, then you have no choice in the matter, but you will still have an increasing Spanish debt, Spanish bankruptcy is not really an option, it doesn’t work the British way.
Your obvious worry will be the safety of your British property and British earnings and whether your Spanish lender can pursue you in the UK. That will depend on your own personal circumstances, but you will be protected by British law which at this stage doesn’t seem very supportive of the aggressive way Spanish institutions operate.
Not that it’s any consolation, but I would imagine your agent being A and the developer TM, who between them have wreaked havoc upon thousands of expats with their machinations over many years. I’ve noticed that the A viewing coaches are on the road again.
I believe there are court actions by expats against both companies, including some in the UK, but I don’t have the details.