Rears are always the first to go, my guess is that they carry a little more weight and as they are in the engine bay they're exposed to more heat.
So rears are always in more demand and harder to find.
As far as I can only see the difference between front and rear units is the alloy socket at the bottom. They are different lengths.
this can be swapped over, the trick (to do this the unit must be removed from the car) is to apply air pressure to the fluid port, fully expanding the lower diaphragm the cone shaped alloy socket can now be removed (it just sort of tucks into a lip in the diaphragm) and the replacement tucked in.
So if you can get your hands on a front unit, you could "convert" it into a rear unit.
I understand that the Trophy models used the same part # units front and rear, these are almost impossible to get now. For those with a MGF Trophy they could perhaps, fit the cone shaped sockets from the Trophy units to the standard model front units.
"Keep calm, relax, focus on the problem & PULL THE BLOODY TRIGGER"
