UK Tourist visa waiting times
One of the most common reason why we see a UK tourist / UK holiday visa refused is because after the first 6 month visa when the applicant arrives back in Thailand they apply immediately expecting it to be much simpler because they have had one UK tourist visa and returned on time and not overstayed their UK visa.
This is not the case because the un written rule for UK tourist visas is one visa one year so your Thai partner in effect can spend 6 months in the UK but then must return to Thailand for 6 months or the British Embassy can use the excuse that your Thai partner is using UK tourist visas to reside in the UK with their UK partner.
Unless it is an emergency then let your UK partner stay back in Thailand before applying for another tourist visa or risk the chance of them being refused, after 13 years of experience with dealing with UK tourist visas we know how the Embassy work.
If you return from a tourist UK visa and wish to apply for a marriage/ settlement visa to the UK then you can apply immediately without any waiting time.