Britain has left-hand-traffic but the driver of the high-ranking British naval officer sits on the left side in the car. All other cars in the film are either clearly marked as non-British or the steering arrangement is not visible and correctly use the left side of the road.
The film puts great emphasis on Rockmouth being a purely civilian harbor and a rather modest one.
The plans to built a major naval base there are kept strictly secret (a major plot point). An official fleet visit by a foreign warship, a uboat in particular, would be not just very unusual but defy the whole secret plot.