My wifes current Thai passport was renewed in Thailand before she recieved her UK passport.

Again I suspect the issue is because she didn't have the exit stamp in Thai passport. At the moment she will have one, it will be enough to indicate it when coming back. As mentioned that would be an issue only for the kids, because she can use the automatic gates.

I'm sure no immigration officer would ever stamp a denial of entry into a Thai passport.

I'm sure no immigration officer would ever stamp a denial of entry into a Thai passport.

That is entire point of asking for it They would let her in on her Thai passport because they would have no basis for the denial.

I actually wrote for the denial to be in writing not a stamp.

I'm sure no immigration officer would ever stamp a denial of entry into a Thai passport.

That is entire point of asking for it They would let her in on her Thai passport because they would have no basis for the denial.

I actually wrote for the denial to be in writing not a stamp.

I understand that but I'm afraid may not work the desired way in the heat of a discussion, giving a chance to the officer to belittle the person for requesting something like that. I totally agree that one has to stand ground one way or another.