The factory location in the engine bay (Cooper S and early 1275GT) is easy and looks like it belongs, but does make access to the clutch very tight to the point where anything beyond routine adjustment, means it needs to come out.
Another place, though more work to fit and not really attractive is above the passenger's feet, inside the car, under the parcel tray. Some of the Works Cars were assembled with them here to keep the engine bay as accessible as possible. Cooperman might have a photo ?
Keep in mind too that the Outlet of the Servo needs to be higher than the Inlet, so that when bleeding the system, you can be sure of getting the Air out. While usually mounted this way, you could always 'adjust' it like this for bleeding, and then fit it back to suit.
There's also an Air Filter on them that does need changing around every 20 000 or so miles, so you may wish to also be mindful of that when mounting.