Peter,
I think you will find you are exempt from R112 rules and regulations anywhere in that vfr shaded corridor as long as you are in it with Southend ATC permission so you do not have to squeeze through a tiny area at the northern end, you have a wider lane.
If you are avoiding R112 rules and regulations then what you must not do is leave the corridor shaded area unless you leave it into the airspace outside R112 (and d138)