These were my hoops 3 years ago. First, Goto local Amphur and ask for their list of requirements. Looks like you're from the UK so you need to prove you're allowed to marry, never married/divorce finalised etc. Make appointment at UK embassy in Bangkok, sit around for a few hours while they sort paperwork confirming it. Not sure of the office names but you need official Thai confirmation too, 3 or 4 offices in Bangkok and 1 in Ubon Ratchathani. Maybe others but not sure. Finally make appointment at Amphur you listed for "ceremony". Good luck.