If you have a Thai girlfriend and you are not married or planning on marrying, you will need a Unmarried visa to the UK, really there isn’t such a visa, the visa you would require for your Thai girlfriend would be a UK Tourist Visa.
A UK Tourist Visa would allow your Thai girlfriend to visit the UK for a short amount of time, within this time your partner will not be able to work and will need to return to Thailand at the end of the visa. Like all UK visas for Thai citizens, the visa application must be made at the British embassy Bangkok