I have a son who holds both a British & Thai passport.When booking his flight to england dose it matter which i use.

I do know he has to show his Thai passport on return.

Also both of my sons passports show my surname on them,and my wife's passport still has her surname on it ,Could this cause a problem, would she need to carry birth cert to prove she was his mother.I will not be traveling with them.

For booking the flight it doesn't matter, as long as you can show the passport. Both airline staff and immigration are familiar with the concept of dual nationality. At check-in always present both passports, to confirm no visa is needed.

It can never hurt to carry a copy of the birth certificate and I would recommend it.

My children (2 thereof) are currently studying in Aust. They have dual Nationality. They use their Thai Passport in and when leaving and arriving in Thailand. They use their Aust. Passport to arrive and depart Aust..