The UK government is already committed to operating F-35Bs for decades to come, and these 12 aircraft replace 12 F-35Bs already planned for in the next procurement package.

The main reason for getting 12 F-35As is for the nuclear strike role and for cheaper operational conversion training for F-35B aircrew. They're not going to do much else, so the inability to refuel from RAF tankers isn't a huge dealbreaker.