If you want an actual service, then talk to the LARS provider for the area. You will then be wearing their squawk and the Solent controllers will see that you are talking to Farnb, Boscombe etc. Then if they want to talk to you they lift the phone to that provider.
The listening squawks do next to nothing for the pilots using them. You get no service whatsoever and no information on potentially conflicting traffic, but in most cases you have to listen to the continuous radio traffic from the aircraft inside CAS. They're exclusively for the convenience of ATC.