When Janette and Mike are in the canoe on the river, Mike is paddling but it is obvious that the boat is not moving. It is probably fixed in a stationary position for the camera shot.
Towards the end of the film Janette and Mike are located between Carlisle (last town in England) and Gretna Green (just over the Scottish border). They reach Scotland by crossing a bridge over a mountain stream. However, in reality this border region is flat, low-lying and marshy, and certainly has no mountains.