Sorry about the slow response. It’s that time of the year. Same goes for all the other questions on the board awaiting a response.
Your mortgage adviser has it right. You won’t find a bank to make you an equity-release loan, though I’m not sure that it’s actually illegal.
Can you find a British bank willing to lend against your property in Ibiza. To be honest I don’t know, but I would be very surprised.
Another way to release the equity in your home in Ibiza is to sell it. If you bought well three years ago I would imagine you’re sitting on some capital gains by now.