Steel blue I was loving your advice untill you hit the line,
"You also need to check if the vat on sale is 21% or 13.5 with revenue as it could be a costly mistake. I think it is only 13.5%"
OP is correctly paying back 21% VAT, so that puts into question the rest of your advice.
Especially this one:
3) Stamp duty will be due on pre vat price for person who purchases.
caracian, please continue to update this thread if you ever get a definitive answer. IN principle I think it makes sense to stay registered, but Im not 100% on the implications of the sale. It was always my intent to deregister before the sale. However I do not have the issue of having multiple properties under the one VAT number.