For a whole host of reasons, UK civil ATC are supposed to advise pilots when the service being provided changes.
If you enter Controlled Airspace from Class F or G airspace, then you can no longer receive the ATSOCAS service you were being given (Basic, Traffic, or Deconfliction). The controller can only give you an Air Traffic Control Service which is effectively either a Procedural or Radar Control service. Note that this doesn't mean that they have to control you with vectors or have you fly at a single level, more generic clearances are normally given.
If you leave CAS, then again the ATC unit has a responsibility to ask you what service you require under ATSOCAS, or to terminate their service and transfer you to another appropriate unit if you need one.
The provision of Flight Information and Alerting services are a given for all classes of airspace.