My experience of the SE corner is pretty similar to Slopey's. Most of it I suspect is people using routings which pass over the Manston to Folkstone area. Everytime I flown through there it is busy, similarly around the GA fields and the 'rat runs' between controlled airspace.
Certainly a gadget would help but obviously is not the whole answer. As for everyone having a transponder? Cannot see it every happening - if only for the well rehearsed Mode S arguments.
Even here in the quiet-ish north it is not unusual to find paragliders, hanggliders and weightshift microlights at typical GA altitudes and they outnumber all of the GA fleet.