Look for the mortgage which is most appropriate for your capabilities and needs. There are a range of mortgages on offer and you should pay special attention to the interest rate and repayment period, fees for setting up the mortgage as well as early repayment and cancellation fees.
If you cannot keep up the mortgage repayments, the Spanish bank could repossess your property in Spain. If the value of the property is less than the total debt outstanding (you are in negative equity), the bank may pursue your UK assets to recover the mortgage shortfall using a European Enforcement Order.
Make sure you fully understand the mortgage agreement you sign. If you have any doubts check with the branch during the 10 working-day period after the binding offer has been provided. If for any reason you cannot keep up the mortgage repayments, you should speak to your bank immediately (before defaulting on repayments) to discuss the options available.