>And why could we not get down from buses, but had to alight instead?<
I believe in the First World War, when London buses were used to move troops
up to the front, the military terminology was to 'embus' and to 'disembus'.
Actually, although I think most Londoners will 'get off' the bus, in parts
of London you can hear people saying (in a West Indian accent) that they are
'coming down' from the bus.