Does my Thai girlfriend apply for a visa in Thailand for the UK or do I apply here?
Firstly you cannot apply for a visa for your Thai girlfriend. It is your Thai girlfriend that is applying for the visa and not you the sponsor. Secondly the visa has to be applied for and issued in Bangkok, so it cannot be applied for in the UK.
You or your Thai girlfriend do not have to attend the British embassy, she may be called for an interview, but you will not be asked to attend the embassy at any time of the visa application.