I was under the impression that THAI would not be operating regular flights LHR until about the middle of 2021 at the earliest.
They may be flying a few cargo or repatriation flights, but there is a complicated list of requirements to be met in order to board any TG flight to BKK.
In any case F class is only available on their A380's or B747's, which they are not using at the moment.
Your best way to contact THAI is probably a phone or skype call to Bangkok.