Annoying but you have proof of the visa etc all at immigration. Go back to them and ask for copies if you don’t have them and then go to the embassy who should be able to sort this and save you returning. After all they are there to represent the British people in Thailand.
for sure but they can’t make that judgement call. If there used to be a child living with you ( or family) then they have to assume support is required. After all that’s one key reason why Thai women find a foreigner to provide some level of good financial support.
I see someone posted the UK government website link. These are your only options. There is a visa with the right to join family which includes spouse or partner. Think this is your best bet and if you have done at least 2 years of the longer term tourist visa should be easier