In fact gogodoc just called as my Thai partner will fly tomorrow. The questions were : have you got a cough? Have you lost sense of smell/taste? Have you got a fever? Have you been in contact during the last 14 days with anyone known to have covid? Have you any other symptoms you want to tell me about?
Then the doctor said "If anything changes before you fly, you must report it"
Job done. We paid £22.30 for the appointment (discounted from £30) and £50 for the certificate, which should be emailed within the next couple of hours.
I work at Heathrow. Our instruction is that passengers are aware of the restrictions, but we don't ask. So long as they are valid for their destinations, we won't be stopping them.