My Wife who has dual Thai/British citizenship will be entering & leaving Thailand on her British passport. Am I right in thinking that she will be able to apply for VAT refund on her way back out of Suvarnabhumi?

Should have no problem, she's a UK tourist.

Even UK citizens visiting UK can claim back UK VAT if they've been out of the country for more than 2 years.

Should have no problem, she's a UK tourist.

Even UK citizens visiting UK can claim back UK VAT if they've been out of the country for more than 2 years.

In almost all countries in the world, citizens HAVE TO enter with that countries passport, even if they own others as well.

I don't know the specific rule in Thailand, but would be surprised if it's different here.

In any case, the UK embassy isn't allowed to help Thai nationals (no matter if they also have UK passport) and for the Thai authorities you are a foreigner if you entered with a foreign passport, so they can e.g. deport you.