Dont know anywhere they are published other than in our unit procedures?
Often they are there to ensure you do not penetrate another sectors airspace who would be unaware of your presence, thereby seperating you from its traffic. ie the FL140 restriction into Heathrow on G1 will provide seperation from Gatwick inbounds from the north on the way to MID, although the receiving sector knows of both aircraft the previous sectors are blissfully unaware of each other. (as long as everyone makes their levels). The restrictions are present at all times, however in very quite periods ie at night when 1 person is working several sectors some of the level restrictions maybe omitted. (eg TC Midlands & TC NW at night the 150 level 25 before may not be required as the same person has control of all the affected airspace.)
Hopefully this makes some sense ??????