Roffa - The LHR point is probably about the 'large' surface area of the zone vs. US airports - A point that is done to death and not that relevant in discussing why PPLs bust clearly marked (if large) zones.
The western edge is consistent with ensuring traffic descending on the ILS remains inside CAS.
The eastern edge probably isn't going to change.
The northern boundary is consistent with ensuring traffic climbing out of LHR and Northolt on SIDs in to the airways system at the minimum climb gradient remains inside CAS.
Same with the southern boundary for LHR traffic, although there might be leeway bringing it north a bit.
You might be able to trim a mile or two in at the corners but that's about all.
Doesn't seem overly stupid to me.