Up to you really Middleton and your financial circumstances and there are many points to consider.
Interest pay rates are probably much of a muchness these days although headlines today indicate UK rates might start to increase as lenders adjust their SVR's although most Spanish mortgages are linked to Euribor which is currently very stable.
Costs are higher if you purchase with a Spanish mortgage as there are taxes, registry fees and Notary fees linked to the mortgage that are independent of property expenses.
UK mortgages tend to be more flexible over early repayment and without doubt lenders probably easier to deal with.
There is no more Title protection buying with a Spanish mortgage against buying for cash. However the one thing I would say is that a Spanish valuation (Tasacion) is far more thorough than in the UK and the bank will go into some of the legal aspects over Title that your lawyer perhaps won't. Many lawyers in Spain dont offer the same service as you would expect to receive in the UK.
IHT exposure (succession tax) could perhaps be a factor but that would depend on what you are considering spending and for the most part should not impact your long term situation. If you plan to move to Spain and maintain the mortgage at retirement any re-introduction of wealth tax might be impacted but again this depends on scale.
I guess that also in the long term you have to decide what you think is going to happen to Spanish property prices and the Eurozone economy. If the Eurozone implodes then there could be an advantage to having the debt secured on the Spanish property as a result of currency realignment - i.e. you have not removed cash from the UK at one level only to find it devalued if it needed to be returned. At the same time, while Spanish prices have dropped drastically over the last 4 years could they fall further if there is further erosion of confidence in the market. If the latter happens you could find you have a debt secured on a property that is higher than its actual value. In both instances we are in the hands of political and economic factors that are extremely hard to predict.
The plan is to retire in 5 years plus......what happens to the mortgage at that time - is the UK property sold and the mortgage repaid? Is the UK property sold and the Spanish mortgage repaid?
No easy answer to your question other than to say it is much a personal decision and for you to try and take a long term view and guage all scenarios. Many people buy for cash in Spain thinking that if they need to they will release equity at some stage in the future if needed. The one thing not to do is to take the view that at some stage in the future you can release equity - that is very very costly and there are severe restrictions in terms of repatriating money and nigh on impossible.