I cannot advise you as to how you should invest your money or whether to sell your UK properties as I'm not allowed to do so. I'm not an expert on the Spanish tax system but if your UK rental income is taxable there, then you will be able to offset any tax you pay on that income in the UK against the Spanish tax liability on the same income under the terms of the tax treaty between the UK and Spain.
Your UK rental will remain taxable in the UK once you have left the UK, if that is what you decide to do. As Belgian nationals, you will continue to be entitled to UK personal allowances as you will see here
to offset against your UK source income which will reduce or negate your UK tax liabilities depending on the level of your UK income.
As non-resident landlords, you will need to comply with the non-resident landlord scheme which you can read about here
From 6 April 2015, gains made from the disposal of UK residential property by non-UK residents will be taxable in the UK, whereas as of now, they are not. However, whilst the detail has yet to be confirmed, it is likely that only the gain arising from 6 April 2015 to the date the property is disposed of will be taxable. The means you will need to have your UK properties valued as at 5 April 2015 since those values will become your cost for Capital Gains Tax purposes. You can see the government consultation document on the subject here
I hope this helps but let me know if you have any further questions.