Much of the irregular shape of UK airspace stems from pressure from various user groups to keep it to the minimum required.
It would be possible no doubt to get more circular as is often the case in the States, but I reckon overall more airspace would be used up in the process.
Cake and eat it, so to speak.
Looking at the NYC airspace, they have smaller surface areas but also shelfs with lower base levels than we'd use here. What significant use a 500ft shelf, or a 1100ft, 1200ft or 1500ft one over a built up area is in an average GA single is debatable but I suppose they can at least say it is there.