As ATCO Two states, the CTA idea helped NATS overcome the objections from several agencies. We didn't want to narrow the "gap" between Stansted airspace in the vicinity of Stapleford, and we kept some unregulated airspace available to the north of Biggin Hill.
The CTA was also introduced at the same time as the change to the low flying rule (now 1000ft over....rather than 1500ft over highest point etc.), thus allowing light aircraft to legally fly below the CTA (as long as they can alight clear of the built up area).
It was a compromise between the ideal and the access issues, and in my experience, people have been able to take advantage of this unregulated airspace.