Many able bodied people are left behind when a bus is full.
Personally, I can't imagine that anyone on a bus would refuse to move for a wheelchair user wanting to board - with the following exceptions:
1) There's already another wheelchair user on the bus. In this case, if there's no more room, then the wheelchair user trying to board would have to wait - just like anyone else.
2) If the bus is so full that people couldn't move away to let the wheelchair user on, thus having to get off the bus and wait for the next one, I'd say the wheelchair user would then have to wait.
3) If there are buggies in the wheelchair area, they should be asked to fold them up (and there should be signs to that effect on the bus) and move. If they don't comply, they could be asked to leave. However, beyond that I'm not sure what a driver could actually do.
I'm sure most wheelchair users are quite understanding, especially when a bus might be rammed, and would wait like anyone else (or call for further assistance).
I can well believe that the bigger problems would come from people with buggies, as in every case of an argument on a bus or train it has been with a buggy, not a wheelchair. That includes one mother who turfed out two elderly people on tip up seats on a FCC service by saying they were blocking the buggy area (and pointing to the wheelchair sticker).
When we travel with my son in a buggy, it's never at the expense of others. He's travelling for free, so he won't take a seat if it's busy. The buggy will be folded, or if there's nowhere to put the folded buggy, we stay with him in the vestibule and move the buggy away from the doors at each stop.
To demand others move away is downright rude, but I've seen it countless times.