I concur - There haven't been any FOS or legal cases or any FSA guidance, which have said that in general ERCs are unlawful, they are not penalties for breach of contract.
They are agreed charges in the event that a borrower chooses to redeem their mortgage during the specified ERC term which is a right the borrower has to exercise if they choose to.
Just because an ERC is a lot of money doesn't make it unlawful or unenforceable. As long as the ERC was clearly stated up front in your mortgage offer, KFI or similar document, it is legally binding and you have to pay it.
The FOS will not support an argument that it's too much and you don't want to pay it.