4468, I suspect that BA were given an incentive to fit MLS receivers on a few 757s(?) as part of the MLS trials - UK flag wave. Other operators did not believe that the ground installation would be continued, particularly as the UK submission was rejected. I don't know why the ground system was retained.
Manchester and Gatwick had trial installations for interference tests and Cat 3 evaluation (plus a low approach over Gatwick electric railway). Manchester considered a weaving 'S' approach to 24 avoiding several hospitals, but this was not flight tested.
MLS would have been able to provide a GBAS type of operation with appropriate on board computing, e.g. parallel runway operation to a lower standard; also a computed GS together using the back course (go around guidance) beam to the opposite runway end.
Another consideration at LHR was to use a computed steep approach GS on the front course to a displaced touchdown for 'STOL' regional aircraft to reduce the wake turbulence spacing. (Reconsidered more recently with ILS?)
Overseas MLS trials used installations at Bern and Tehran. Steep approach and segmented approaches were demonstrated at Bern with 4 deg autoland; also take off and GA guidance was tested towards 'the close in hill'. Bern also provided an offset MLS for a cross field grass runway towards the ground station, as well as maintaining the full 'ILS' on the paved surface.
There were associated trials with helicopters, but I don't recall where. Some of the ideas would have enabled multiple approach paths and GS to platforms and one idea was for simultaneous helicopter IFR approaches to LHR from the South with a landing zone where T4 is.