No, it's quite simple - if there's space you board. If the
buggy/wheelchair space is taken, you wait for the next one.

How can you tell from the front of the bus?

You wait for the bus to stop first to allow for the possibility of
passengers with pushchairs alighting, and then you look through the doors
and windows. They tend to work quite well for that sort of thing ;-)

Although the drivers on my local bus route often seem to sail past
compulsory bus stops half empty presumably thinking they are full, their
knowledge of their loading becomes encyclopaedic when it comes to pushchairs
and buggies. So as soon as they pull up they'll let you know if they want
you to fold the buggy, use the middle doors or in exceptional cases not come
on at all. OK so some will only allow two buggies on even though the bays
can usually cope with three given cooperative fellow passengers and the more
modest size buggies and the bus is otherwise empty, but most are a bit more
flexible.

Going back to the OP, I was surprised to. Their are minor issues with the
design of buses and with a minority of drivers, the main problem is actually
the other passengers with buggies, and in particular those who don't use an
appropriate pushchair or buggy for public transport.

G.