When the BT telephone in the red phone box rings, it rings with an American (US) ring style. The phone rings with a long ring followed by a short pause then a long ring. In the U.K. the phone would have a double ring followed by a longer pause then a double ring. That all being said, the particular BT phone in the phone box wouldn't even ring with a bell, as heard in the film, but has an electronic ringer.
Prisons in the U.K. do not require all prisoners to wear a uniform. The majority will wear a uniform for the first 2 weeks only, and this consists of a grey tracksuit as opposed to the striped uniforms (complete with hat) in the film.
The goof item below may give away important plot points.
The police tell the Brown family Paddington was caught "red-handed" stealing the pop-up book, and he is convicted of the crime. Yet neither during the chase or his arrest is the book ever in his possession.