I would make the phone call myself in that situation, there would have to be a reason why the Pilot was unable to advise the change of frequencies to Shoreham, and then to subsequently have words as the pilot inadvertently entered their CAS does seem a little churlish.
Of course there are always two sides to every story, but given the same circumstances , as an ATCO I would have ensured we let Shoreham know of your change of frequency.