The Equality Act states:
"A person (P) has a disability if—
(a)P has a physical or mental impairment, and
(b)the impairment has a substantial and long-term adverse effect on P's ability to carry out normal day-to-day activities."
There's no mention of wheelchairs at all.
"'substantial’ is more than minor or trivial - eg it takes much longer than it usually would to complete a daily task like getting dressed
‘long-term’ means 12 months or more - eg a breathing condition that develops as a result of a lung infection"
There is a full guide here:
odi.dwp.gov.uk/docs/wor/new/ea-guide.pdf
"A man with achondroplasia has unusually short stature, and arms
which are disproportionate in size to the rest of his body. He has
difficulty lifting or manipulating everyday items like a vacuum
cleaner, or bulky items of household furniture, and has difficulty
opening moderately heavy doors, and operating revolving barriers
at the entrance to some buildings.
It would be reasonable to regard this as a substantial adverse effect
on normal day-to-day activities."
So basically if you are not physically capable of folding your pushchair, because of some physical limitation, then the bus company should not turf you off, and indeed to do so would be discriminatory.