IO540 wrote: [snip] "This summer I have seen gliders which were definitely a few hundred feet in the 2500ft London TMA (around MID)."
Illegally, I presume.
There are or have been places and occasions where gliders are permitted in certain localised areas of the LTMA, e.g. during certain competitions at Booker with CAA special approval and then of course NOTAM'd, but in general gliders have been banned from the LTMA since, IIRC, 1975.
For the most part, the LTMA will be as free from gliders as it will from illegally intruding GA aircraft of other kinds; just as assigned levels will be free from anything else except level busts or people in the wrong place due to pilot or ATC error. Breaking the law and making errors are, unfortunately, human characteristics which seem incapable of complete eradication.
Chris N.