Go to your local immigration office and extend. The cost is 1,900 baht. If you don’t then IF you get to the airport then you’ll be charged 1,500 for a 3 day overstay.
Some would question why pay 1,900 baht rather than 1,500 baht. But here goes.
An overstay stamp in your passport is rarely ever a consideration for coming back to Thailand. However other countries may see it as an issue if you apply for a visa for theirs.
If you were to be stopped by a police checkpoint whilst on overstay then a whole can of worms can open up including deportation and a ban from coming back to Thailand.
The latter would rarely happen. But it could.
Never plan to overstay. Get the extension and cover your ass. It’s just not worth the potential problems you could have if you don’t.
You don’t need any documents from Thailand to apply in the UK. You can’t apply based on marriage and you can’t apply based on Thai family as your daughter is too old, but as you are over 50 you can apply based on that.
Check the London embassy website for specific requirements.
Within 24 hours and leaving via an airport you’ll get an overstay stamp in your passport but they’ll usually waive the 500 baht fine.
The overstay stamp isn’t an issue for returning to Thailand but can cause issues in other countries when applying for a visa for there. They see the overstay stamp for Thailand and may it cause problems.