What is the likelihood that anything would come of not telling the first station (Shoreham in this case) that you're changing frequency?
Well, according to my Air Law text book, whatever else was going on an Alerting Service was in effect. If an aircraft receiving an alerting service simply disappears from the frequency then the ATSU has to try to find them, ultimately ending up with Search & Rescue.
If they can find the time, I suppose.
And anyway, does what it says in the Air Law text book actually reflect real life?