You can be charged yes, it’s a bit of a grey area in the law. Fees are capped at £50 unless the agents have additional ‘reasonable costs’ eg it costs them more than £50 to change it over.
The issue is, it’s not just ‘change a name on a contract’ they have to get an entirely new one drawn up to reissue it. Ask them for a breakdown of what it costs them and speak to citizens advice but if that’s what it cost them to change it all over they can charge that to you…