I wonder where the mother is, as there is a court order in place it makes me think she is in the uk, you should get advice from a solicitor as I doubt you would be able to take the kid out the country without the mothers permission whether he is duel citizen or not
they normally are classed as your hold luggage with nothing more to pay. And definitely don’t need to pay import tax if you are on a golfing holiday. Just check the dimensions and weight. As long as you are within both you have nothing to worry about
It’s not as simple as having money in the bank, you have to fill one of the criteria too, like a remote worker and I would believe the money would need to be in your countries account not Thailand. You have to prove you have an address in the uk too. And if you go for remote working a copy of the contract from your company. As you have been retired already in Thailand for 8 years you might find it difficult to then justify that you are working. Good luck 🤞