@Pa**
, I don't know who told you thaiz can't have multiple passports when they get older but they most certainly can. There is NOTHING written in thai law that prohibits it. All it says is IF asked (which no one has ever been) the thai will pick on nationality.
I know a thai who holds a thai, an aussie and a brit passport and has no problems flying out on his thai passport and entering another country on one of the other passports.
When you check in show both passports to the airline (so they know you don't need a visa in the thai passport) but when you stamp out, just present your thai passport. Then when you get to where ever, just present the UK passport.