The default position in Thailand is that the mother has custody of any children in the event of a break up. However, a father can challenge that and given that the child's mother is in agreement, I can't see (but you need to check) any reason why a court would not grant you custody provided you are considered to be a suitable person.
However, I don't think there is any absolute 100% cast iron guarantee that things couldn't change in the future. From what you say its doubtful that anything untoward would happen but there may be a way that the mother (having seized the child) could go back to the court and claim she was acting under duress, not well at the time.....any number of other bizarre reasons. Remember, you would be dealing with Thai courts in Thailand where strange things can and do happen.
I doubt there is anyone here that can tell you with any certainty what the position is under Thai law so I'd recommend you speak to a Thai lawyer.
One thing that may offer you some protection and there may well be some legal advice on this forthcoming, would be that the child gives up her Thai citizenship and therefore becomes solely protected by her UK Citizenship. meaning the UK may be able to request the child's return if the worst came to the worst. However, I'm not sure that an 18 month old would be able to give up her Thai citizenship.
Personally I'd get advice from a Thai lawyer and go for custody.