OK...
I work in the TMA and I certainly use expedite to help me do my job efficiently without using excessive vectors in small and congested airspace.
Here is what I expect... I LOOK at what the a/c is doing.... I JUDGE the conflict..... if I decide there is enough space, but that an expedite for 4 or 5 thousand feet would make it more comfortable, then I ask the pilot to expedite. I EXPECT a higher climb rate than what he is giving me before the request and I EXPECT him/Her to tell me if they are not able.
It is not a form of separation per se, it is a tool used by skillful controllers who understand A/C performance.
As stated, Many A/C offer a good rate on departure (I know 'good rate' is a moot phrase but in this context it is obvious what the pilot means)
For a EGLL Dover departure, it is not always any use because of the stacks, but on a Lambourne, with such tight restrictions, it can be a godsend. However, being intelligent, I do not always take up the offer if there is too much traffic in the way. In the same way that pilots are intelligent enough to know that if I do not use it it is because I cannot at that time.
They also know not to query why I am not using it because they know I am busy.
It is about COMMON SENSE, a thing that a few of the pedants around here could do to remember.
Stay safe
