rated U - with a knife held to a throat!

Indeed ! It is also very unPC at times. Strapping down a beautiful woman to a chair and then 'trying' to torture her to get the necessary information.... For a supposed kid's movie it's also pretty violent at times.
And except for school and church, kids in those times seemed to enjoy almost complete freedom and roam the streets at will (in this case a bombed-out London from the Blitz and the V1's)

Yes, it is actually quite a violent film, and some of it could be quite scary for younger children I would think - i would expect it to be a PG nowadays.