the aisles in the buses are usually big enough for a buggy to go through - until the bus design decides to throw in a pole at the end with a curve that stops it (not at the wheels but somewhere else along the frame)! The P4 from lewisham is like that.
that is why people board from the middle doors.
also, passengers refuse to move down the aisle so you cannot push your buggy to the back even though the space is there, which also leads to boarding thru the middle doors.
i have been on plenty of buses where the wheelchair and buggy space is taken up by shopping carts. who should get priority then - the children or the shopping.