You need to consider the mortgage interest rates versus the mortgage set up costs, also the reduction in CGT and Inheritence tax in Spain if you have a Spanish registered mortgage. You will only pay tax on the actual amount owned. (sale price minus mortgage amount).
You will also pay less tax on your 0.2% wealth tax each year as the same principle applies.
It would be easier to control in the UK if that's where you live, however if you take a UK mortgage you would need to transfer a large amount of money to Spain and the Pound/Euro exchange rate could effect the amount you need. Look at a company such as www.currenciesdirect.com
I would always ask any Spanish mortgage lender if you can make capital payments off your mortgage at any time without penalty. Then you could transfer money from the UK if the exchange rate improves.
Hope this makes sense.