Unless you are running it a a registered business, you won't be able to reclaim anything against your personal tax. On the UK tax return, you state how much tax you have paid abroad and if it is less than that due in the UK, you are liable for the balance. If the amount paid is more than in the UK, you may be able to reclaim the difference.
In Spain, as a non resident, there is no allowance against rental income to off set mortgage interest, depreciation, insurances, etc. In the UK you can set against these allowances, so you will pay your Spanish taxes. As a consequence, you shouldn't be liable for anything in the UK, we haven't been yrt!!