In ATC a little knowledge can be worse than no knowledge sometimes. Unfortunately NATS, being the monopoly Area service provider, seems to write peculiar rules and instructions daily. The bulging offices are awash with wunderkind ideology. The increasing [en route] use of "minimum clean" takes no account of the savage fuel penalty. The latest "270knots" instruction is no more than a band-aid solution to chronic "full stack" scheduling. The only capacity/movement gains in the last 5 years have been a result of chipping away at the already reduced separation criteria.
[Tongue-in-cheek of course.....I would add that this is why UK uses different air to the rest of the world.....this enables a completely unique set of wake vortex separations to be used.]