CAIT (the automated infringement alerting system in the London area) will highlight any unknown traffic with A/C that is flying at or above the base of CAS.
So if the base is 2,500ft, any unknown traffic with a Mode C readout of 2,500ft or above will be highlighted. Tracking along at the base level is very unlikely to result in you being imprisoned for infringing! Above the base level ATC may well have to start avoiding you as we might have traffic inside CAS at just 500ft above the base, but we're well aware that Mode C tolerance is +/-200ft. Better safe than sorry though.
See
AIC 15/2007 Pink 112 and
AIP ENR 1.6.2 where it is strongly recommended that in airspace where a transponder is not mandatory you should still select A
and C.