I don't think the UK will let go of them... Strategically, Scapa Flow is incredibly important and played a huge part in both world wars.
This is a quite clever publicity drive to get some more money, nothing more. Good luck to them - although I recently visited and the roads were in better condition than those in my county in England, and the ferry services (Except Pentland was still down) were excellent. Quality of living looked (admittedly, through the eyes of an outsider in summer) pretty good too, certainly equal to similar rural parts of Cornwall and large areas of Wales)
At this point, I wouldn't be surprised if we just decided to split the UK up to its shires. Each operating independently and requiring a passport to travel through.
Weren't they technically only on loan to us, and supposed to be returned - but as with most things we "borrowed" we kind of forgot the islands weren't ours?
Shetland has its own independence movement, along with one nut job who has declared the islands to be under occupation, declared himself president and tries to get away with not paying road tax on his car on account of it being a diplomatic vehicle.