My husband and I have made the hardest decision ever, and that is to return to the U.K. after living in Spain for 14 years. We have bought a property there in June 2018, however, we had accumulated properties and vehicles here therefore 2.5 yrs into our 3 year repatriate plan, we still have 1 property left to sell, so we are currently in limbo.
My questions are:
1. Do we need any paperwork from Spain to present to the U.K. govt? If so, what and where would I would I find them.
2. Is there a waiting period before we are able to register with the NHS etc
3. Is there a system whereby we can pay capital gains at source here in Spain, as we would prefer to close all bank accounts as soon as possible

Any advice on a stress free move is appreciated, thank you!

No, again presuming both you and your husband are British Citizens then you are entitled to free NHS care the date you return with the intention of taking up residency in the UK. You may struggle to register with a GP until you have proof of address, but you are able to get healthcare the day you arrive.

will leave this one to others more au fait with tax matters.

Lots of planning, which you are already doing. Good luck with the move.