mm_flynn,
There aren't likely to be any non-emergency, non- military heavily loaded heavies operating a climb profile in the UK outside controlled airspace, however, his point is similar to points UK ATCOs have previously made about why cutting a couple hundred feet into the bottom of LHRs departure airspace is a problem (which is not because aircraft are often scrapping the bottom, but sometimes they are - plus the obvious issue of 'it is against the rules')
The only departure that these days is likely to be close to the bottom of CAS on departure from LHR (barring unusual circumstances) is the A343 or A346. Everything else goes up at a reasonable rate.
Around LHR it is 09L/R arrivals that are far more likely to be embarrassed by anyone marginally inside CAS as, out to the west where the CAS base is 2,500ft or 3,500ft, the LHR arrivals are routinely only 500ft above said base altitude.