first off, outer and middle markers were at one time used with standard beam approaches, even before ILS approaches were widespread.
inner markers were and are still used in CAT 2 apchs.
Middle markers may also trigger glidepath extension in certain planes.
did your particular plane have a small switch indicating HI/LOW?NORMAL?
flip it to high and you will hear it better...it is also helpful in using this switch to know your position. high warns you that you are approaching the marker, then switch to low/normal and start your time when you hear/see the flashing light again.
the high low isn't volume, its sensitivity.
as to the number or location of marker beacons in england...why not ask each control tower you are near or radar facility? or ask an instrument flight instructor.