For something to be a classified as a terrorist attack there has to be a recognised political and/or religious ideology attached to the motive and a provable allegiance to a group formed for the specific purpose of targeting the community or the state or the organisation they see as their enemies. Otherwise it's just one arsehole attacking someone or something they don't like.
As far as I know there are no formally recognised terror groups in the UK at the moment that target mosques or muslims. And if there are, then the police have not been able to make that link in this specific case.