Could I ask what is the situation when the bus is neigh on full when
arriving at a stop, so that already there is not sufficient room for a
disabled passenger to board?

Are some able bodied passengers asked to leave or is the disabled out
of luck?

AFAIK the low floor buses are to provide EQUAL travel rights for disabled
passengers.

Therefore in my experience if I am driving a bus which is full and a wheelchair
user wishes to board, then they have the same right as any other passenger.
They have the right to wait for the next bus.

(Not being nasty just realistic) :-)