I remember that the Swindon and WCO corridors were at FL230 &FL240 based on BDN-BZN-I can't recall the eastern end. They were installed to allow mil a/c to route from East Anglia to the South West without formal airways clearance. They were activated with I think 10 minutes notice. I am sure of the lower level because Bristol inbounds were coordinated from LMS to BRS sector at FL220 below the corridor. Similarly with Birmingham inbounds to DTY sector.
That's all I have to say on this.