Thank you all for your most helpful responses
So, just to confirm my understanding:
ATSOCAS (as the name suggests) is outside controlled airspace, and it is in this airspace where one can obtain Basic or Traffic Service (or Deconfliction or progressive, but I shouldn't need these for my current operational needs).
ATSOCAS only applies when flying VFR in Class E, and also in Class G
If I went into controlled airspace, such as Southend Airport, I would just make a call *(before entering), but wouldn't request a service, so to speak. i.e "Southend Approach, G-ABCD". Then await a response, nothing about a service. Previously, I'd been told to do "Southend Approach, G-ABCD, request Basic Service".
A listening squawk is a great idea for busy radio. Simply set a Squawk code (defined by the frequency you're listening in to), and don't make any calls. The controller knows you're there, and two way comms can be established if necessary
If you feel like experiencing a little busier airspace, PM me, and I'll be happy to show you what it is like out there - maybe get a zone crossing or two! (I fly out of Biggin Hill)
If that wouldn't be too much of a problem Alex, that'd be really appreciated, thank you very much.
Thank you all once again