Re: Prestwick??
Machrihanish (Campbeltown) is not mentioned, but it already has a bigger USAF built runway, and is more remote. And being an airport, remoteness is not an access problem.
However, depending on the next referendum, it may end up as a Baikonur style lease.
Other options, Woomera is being developed again, and also Ascension Island has an appropriate name and location.