Can a Thai national with both Thai and UK passports enter the UK on the UK passport and reenter Thailand on their Thai passport?

I don't see the point in keeping this post open. It's not quicker at immigration if a person has a Thai passport or Thai national (whatever "quicker" means, I do not know).

If a person enters on a foreign passport, they are treated as a foreigner for purposes of immigration, and that includes needing an extension of stay. If a person wants to swap passports, they would have to leave Thailand by air, and re-enter on the new passport.

With the thai Passport immigration must let the thai national in. Thai national it is by Blood given

YES

That is the normal method. If the Thai person enters Thailand using their foreign passport then they are treated as a foreign person and need a visa and are subject to 90 day reports etc.

Yes they can, but they need to change passports by air and not by a land border (entering by land is a moot point anyway since the land borders are closed)