It's not a matter of citizenship but residency. If you're resident in the UK (whatever your citizenship is) you can use your driving license abroad during holidays (international standard is 90 days). If you become resident of another country, you're supposed to exchange your British licence with a local one and you can use it in the UK if you come for vacations. The good thing with Thailand is that they don't keep your British licence like they do in many county (ie Canada, Australia, USA, all Europe) when they give you the Thai one.
According to international agreements, you should get a driving licence of the country where you live. That's why you can't get a British one with a foreign address.
it happened to me once, the boss was at a meeting in Bangkok and couldn't sign the extension stamp. They stamped under consideration 15 days. He came back the following day and I got the 1-year extension.
Thailand SHA Plus refers to the certification issued by Amazing Thailand Safety and Health Administration and their "Sandbox program".
Phuket Sandbox started officially, vaccinated travelers can arrive directly to Phuket, and stay there without quarantine starting from July 1st. All vaccinated tourist must stay 15 days in one of the SHA Plus certified hotels, where at least 70% of the staff have been vaccinated.