KerryB
You will not lose you nationality if you spend more time here in Spain but if you spend more than 183 days per year here you will be fiscally resident in Spain and therefore have to fill in a Spanish residents tax return. (As you own a property here you should be currently filling in a non residents tax return which is based on your property value)
Be aware that if you are fiscally resident here in Spain you will have to declare your worldwide income and assets. This year saw the introduction of the model 720 where you have to declare assets of over 50k. If you sell your uk property and keep the money in a bank account, wherever it may be held, if it is over 50k you will have to declare it and pay tax.
I would suggest you get advice from a tax accountant before making any decisions.
Hope this helps.
Daniela