Yes, UK passport holders have to use and agency and you keep hold of your passport until the new one is sent back from overseas. I got mine back from the UK in under 2 weeks. Which was nice.
Yes I thought that may be the case but I thought it was a rule in Thailand that you cant be without your passport. A rule made by the Thai government. I just did mine for a replacement, the agency that deals with UK passports voided the old passport when the new one was sent back. I thought I had to send my old one but no, I was advised I have to keep it. Just had to photocopy every page etc. Good luck.
I may be wrong I have a UK passport I didnt have to send my old passport off I held onto it until the new one returned. I thought you were not allowed to be in Thailand with out it. Immigration will transfer the visa over I used the one at Central Festival.
It can take a while to get all the documents together, good idea to get things moving before you arrive back. Or apply while you are outside of Thailand.
I do have the correct visa, but due to covid I moved back to Bangkok. I was living in Pattaya where the visa was issued. My TM30 is a Bangkok address . Thanks for the information.