When you fly out, it's the destination's rules that obtain, ie. Thailand's. But you will also need a negative PCR or antigen test result before you return to the UK - a 48 hour one as opposed to the 72 hour one required by Thailand. Message KLM here. They will respond:
I had everything done too! No rejections nor resubmissions required in the application process. I had everything printed. Twice. That wasn't my problem. I'm just telling you what happened despite my excellent preparation and planning. Even Emirates didn't know in retrospect that I'd flown on three, as opposed to the two flights I actually booked. It's a shit-show.
I found the whole process of uploading 12 documents for the visa a real onerous pain in the arse. There seemed to be some confusion about the PCR, RT and Fitness to Fly element of the PCR test and I knew that a friend had seen a woman without a Fit to Fly being denied entry to her flight so that didn't help. I couldn't scan my QR code and neither could my friend so that created more uncertainty. And I knew that Qatar had denied entry to others whose codes didn't scan. When I got to Heathrow to check in, I was told by Emirates that I couldn't stay in Thailand for over 45 days. This was incorrect. I could. I was told that my ASQ hotel was only an AQ hotel and therefore didn't meet the criteria. It did meet the criteria and I am there right now. I was asked for a receipt for my PCR even though I had the PCR certificate. I was meant to get two flights to BKK. However, the second plane also stopped at Phuket and I had to wait for other confused passengers who'd accidentally disembarked to get on again. Honestly, the whole frigging trip was a nightmare from start to finish. I'm now in quarantine in BKK til the 11th. I'm in recovery!
This is unhelpful advice. Adequate insurance is a necessary to enter Thailand. And the Embassy will check all passengers have it before issuing a visa. I have just flown in to BKK from London and it was a nightmare - and I had ALL of the right documents.