Here, a London suburb but on the outskirts at the time, and still including some sparser areas of farmland, there were electric call points at various places -- a "lollipop" consisting of a break glass alarm button (I think) on a short pole. The volunteers were called out by firing maroons, just as always depicted in films about coastal lifeboats. The horses were supplied by the bus company, presumably those resting between shifts.
Caveat: written from memory of the old records which I haven't been able to make time to re-read.