The last Royal Yacht Brittania, in theory it had a war role in reality it was never deployed to The Falklands or even for used in disaster relief.
Partly because she was fueled by bunker fuel, not marine diesel, so she would have need her own RFA tanker. She also had other technical issues, such as the submarine engines used to generate on-board 240v. By the time she was paid off they were causing major problems.
Add to that HMY Britannia was not very large, and not a great sea-keeper, so it made far more sense to take up more recent vessels from the trade.