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.

There is no need to heat formula in a bottle warmer. Formula are always made before a baby's feeding time. If baby prefers a warm temp fr milk, add more hot water to the bottle before adding room temp water to bring down the temp. There is no need to a bottle warmer for formula and formula have to be finished in an hour. Formula can not be made in advance.

You would usually make milk at the time baby needs it. So just mix hot water with room temperature water. No need for bottle warmer.