Hi! I think it would be easier and safer to apply for the DTV before leaving the UK.
Let's not forget that a visa is a privilege and not a right.
You can meet all the requirements and the Kingdom of Thailand denies you a DTV - which is its right. Check this group and you will see applications are being denied even when every requirement is met.
If that happens, what will you do? Border runs aren't a viable solution. ED visas are being scrutinized and despair can quickly push you into the arms of scammers.
If you intend to stay long-term in Thailand, save yourself some useless stress and get you DTV before leaving.