It is basic requirement that a Male party needs to show proof of support for the female party. If the Male party is Thai, he also needs to show that proof.
You can show them the letter from the UK embassy explaining why your income was not stated in the affirmation and then show them your payslip or anything to support the OCCUPATION stated in your affirmation.
The check in counter will open 12:30am so I don’t think you will be able to pass immigration before midnight. Don’t worry, if you pass immigration around 1 or 2am you won’t be paying an overstay fine.