Unfortunately, there are a lot of homes out there (even ones who say that they specialise in dementia) who only want the early, easy stages of dementia. Then once the resident progresses to the more advances stages, they will be asked to leave. Do you know what particular behaviour has triggered this?
There is no point in trying to fight it - you are right, they probably dont have enough staff who are trained and have experience of dementia. If your dad is LA funded, then get onto Social Services and explain that he has been asked to leave. If he is self funded then Im afraid that SS will just leave to look for somewhere yourself.
When you go and look at care homes, look beyond the decor and furnishings at the actual care given. Mums care home was shabby and old fashioned, but the care was wonderful. Look to see whether there are residents with very advanced dementia. If there are, then this usually means that they will know how to care for your dad when he reaches that stage too. Also, talk to the manager, be brutally honest about what your dad is like, ask them how they would deal with him - woolly answers usually mean that they have no experience - and ask what sort of behaviours they would not tolerate. This will give you an idea of whether they would ask your dad to leave at some stage as well