OK, you lured me in...
We had a note put in our ops file recently about this very point, instructing us (for the time being) not to issue block level clearances to any aircraft (including military) in Class E.
As far as I am aware this has not extended to any other group, is not the subject of a TLI or National Instruction.
It might also have something to do with recent instances where military traffic has been in the vicinity of VFR at Flight Levles (think of the speed!) with the standard 500' as per standard operating levels.
I think somebody, like you, also raised the MATS reference and yet we were told no block levels in E in training - for the obvious reason that it potentially eliminates the default safety mitigation of the airspace which is the table of cruising levels.
I shall endeavour to seek out the source of the information....
