It’s fairly straightforward. It’s just like any other sale, but you’ll be paying in pounds in the UK, right?
First agree a price in £, convert into € using an appropriate exchange rate (be aware that the £ is currently getting stronger against the €). Then get a banker’s draft or pay into an escrow account (or whatever payment method you choose), and take proof of that to the notary on the day of signing the deeds. Otherwise the process is just like any other sale, with one important exception. If you are UK-resident (which it sounds like you are), the buyers will have to pay the Three percent Capital Gains Tax Retention on property sales by non-residents, so that means 3% of the price you agree (in €) will have to be paid to the tax authorities in Spain in €. So you can only accept 97% in £ in the UK.
You’ll need a gestor or lawyer to help you in Spain. I don’t know how well you know your neighbours, but make sure you’ve got payment guaranteed (like a banker’s draft) before you sign over your property to them. Always better safe than sorry.