Factual error: Foyle is subordinate to an Assistant Commissioner of the Metropolitan Police, but Hastings had its own force (merged with other Sussex forces 1943-47) and the Met had no jurisdiction outside London.
Factual error: Sergeant Brooks calls Det Sgt Milner "Sir" but they are of equal rank.
Factual error: Foyle is a Chief Superintendent. But the series is set during the Second World War and the rank was not introduced until 1949.
Answer: They are using the accepted names of the bombs. During World War 2 the Germans used the metric system and produced bombs ranging from 50kg to 2500kg. Referring to the weight of the bombs in pounds (or fractions of tons) risked confusing the situation and would require someone to understand how to convert from imperial measures to metric, not an easy thing to do when under stress.