The best way to hedge your exchange risk is to match the currency of your new asset with the currency of the liabilty created to finance the purchase.That is to say,obtain a mortgage in euros against the security of your Euro asset and do not create an asset/liability currency mismatch by borrowing so much against your UK property.
If Sterling strengthens against the Euro then your foreign asset is worth less but this is mostly compensated by the reduction in the Sterling value of the mortgage.You further benefit from a reduction in the Sterling servicing costs.
Obviously the converse is true but any profit made with Sterling weakness will be off-set by an increase in the sterling value of the mortgage and the cost of servicing.In summary your exchange exposure is limited to the net equity in the property (ie. Your cash deposit) and the exchange fluctuation on your annual motgage repayments.
This takes away the the temptation to gamble on the macro-economic conditions in one currency zone being better or worse over time than that of the other (eg Euro interest rates versus Sterling rates; house prices in UK versus Spain: inflation differentials etc.).I assure you very few ever get this right and if they do it is more by accident than design!