It's perfectly OK for non-citizens/non-residents to own property in the UK. (Indeed, many newly-built properties in London are snapped up sight unseen by absentee foreign buyers; it's gotten to the point where developers routinely market new developments to buyers in Asia before they go on the market in London.)
I presume you plan to rent your UK property to tenants after you leave. A major concern for this is probably going to be the availability of mortgage finance. Assuming you still owe money on the property, you'll either need to get permission from your current lender to rent out the property, and/or refinance onto a buy-to-let mortgage. When the time to refinance comes, you may well find that lenders willing to make buy-to-let loans to expat/overseas landlords are few and far between. An independent mortgage broker may be able to help here.
Tax implications for renting out a UK property as an overseas landlord:
- You'll be liable for UK income tax on rental income; if you're a non-resident landlord, your estate agent will withhold UK tax on received rent.
- When it comes time to sell, you will be liable for capital gains tax. Though if you've lived in the property previously, private residence relief may reduce or eliminate this, as you won't have to pay a pro-rated portion of the capital gain corresponding to the time you lived in the property plus three years. So for example, if you live in a property for four years, rent it out for four years, then sell, you are exempt from seven-eighths of the capital gains tax.
Implications for savings - should be the same for a non-UK citizen leaving the UK as for an expat Brit:
- Pensions: You should still be able to draw benefits from a UK pension once you reach pension age. These will be subject to UK tax, same as if you were UK resident. You might be able to transfer a UK pension balance to a non-UK pension scheme that meets certain criteria, but this is a complicated subject so take professional advice before doing this.
- ISAs: Non-residents can't contribute new funds to an ISA. You won't need to cash out ISAs you already have.
- Other (non-ISA, non-pension) savings: Interest earned in UK bank accounts is still subject to tax, though the Personal Savings Allowance coming into effect in 2016 should make this a non-concern for most people.
It's pointless to speculate about what happens in the event of a future UK exit from the EU, as we don't know what the terms of any such exit would be. Note however that all the above points are largely independent of the other country concerned.
One last caveat: the above answer deals with UK tax law; I know nothing about Spanish tax law, though if they follow the practice of most countries* they are unlikely to tax you on overseas income you don't remit to Spain, and even if they do you will probably be eligible for foreign tax credits (which would reduce your Spanish tax by the amount you've already paid in UK tax) or some other form of relief on double taxation.
(*Important exception: The USA considers US citizens and permanent residents liable to US tax on all of their worldwide income and gains. Speaking from personal experience, much of the above advice will have special caveats from a US perspective. Seek professional advice if this applies to you, and/or avoid being American if at all possible. :-))