I would be surprised at a nursery being registered without having outdoor space.
Childminders however are in a different category - they may live in a flat. However they would arrange outings to a local park, or other suitable place.
An outing on a bus for a child can be exciting. However by the sounds of it, in the OP's case that nursery is doing the trip regularly. Maybe it is a problem with crossing roads, maybe it's down to the distance - a few stops on the bus may well be several miles (would be in my area).
The number of adults:children sounds as though they are working to minimum standards for indoors... so not taking account of the increased risks being outdoors. I would wonder how they would cope if a child had an accident on the trip.