The owl seen in Bridget's garden is a barn owl. These owls do not live in the city and it would be extremely rare to see one in London, let alone visiting a garden. The correct owl would have been a tawny owl.
The children's school appears to only have one member of staff. Mr Walliker is the science teacher, but he also does gate duty alone every morning, runs the Job Day, runs the Outward Bound trip with only the help of parents, and single-handedly runs the music concert while accompanying on piano.