My baby is used to drinking warm breast milk. Is it okay to heat formula in a bottle warmer?

Agreed with Jorelle. I don't see a need for you to use a warmer if you are giving your baby formula milk as you would have to make "upon order". As per the back of the tins, there is an instruction to discard remains after an hour of making it as room temperature would encourage the growth of bacteria it in and giving it to your child may give him stomach upsets.