When you are in a relationship with a Thai lady, if it is her first time to the UK and you do not plan on getting married, the best visa for her to apply for is the UK Tourist Visa.
When she arrives in the UK she will have a date when the visa expires and she should return to Thailand. Do not allow your Thai girlfriend to over stay her visa as this will be held on file and will surface if she applies for a UK visa again. Over staying a visa can result in a 5 year ban from being able to visit the UK.
Thai girlfriend renewing a UK Tourist Visa
Your Thai girlfriend cannot renew or extend her UK tourist Visa. When she returns to Thailand she will not be able to apply for another tourist visa for 6 months.
When her tourist visa expires, your Thai girlfriend will not be able to renew or extend the Visa. The only option available will be to return to Thailand. Wait at least 6 months and then make a fresh application for a New tourist visa. The UK immigration department will not issue two UK tourist visas in one Year and if you apply before the six month period she will be refused and any refusal will resurface when she applies again.
The only other option available to enable her to return to England within the six months is to apply for a UK Fiancee visa. You should only apply for a UK fiancée visa if you plan on marrying your Thai girlfriend in the UK. If you apply for the fiancée visa and she returns to Thailand without getting married it will create problems if you apply for another visa.