You could travel all around the EU for years and not get a stamp... they are not really looking for stamps of other countries, just their own. Just out of interest, how can someone have 2 British passports?
If you are not actually planning to use the flight but want to be safe, I found Krabi to kl flight for 600 baht recently. Pennies. Takes care of the issue.
If you are extending in Bangkok, you will not be asked to show proof of onward flight. Probably true in any immigration office but 100% NOT asked in Bangkok. You’ll be ok.
It’s the service of crossing and stamping in Myanmar. It’s not a fee to stamp into Thailand on 30day visa exemption. A bit expensive but it’s a nice little monopoly.