When Hyacinth and Richard open the door to the cabin after they dock, the bed is flat and nothing is on it (besides the cover). A second later, the bed looks more cushioned and 2 robes are on it.
The locks on the door of their cabin do not match from interior to exterior shots.
If you look out the porthole in their cabin you can see light reflecting off a clear plastic shower curtain. That means that their cabin was probably shot on a sound stage. That would also explain the miss matched door locks.
When the car is stuck in mud in the field Hyacinth is going to the front of the car to push, as she is doing so the full view of the car is shown. The front wheel that has been spinning in the bog hole is visible but there appears to be no mud on the wheel and the Tyre is dry.
At the start of their long car journey to Southampton the Redbridge tower block is visible through the rear window. In fact Redbridge is in Southampton and only a short distance from the departure terminal.
When they are supposedly waiting for the QE2 in Copenhagen, they are actually in Oslo. Several landmarks can be seen, and most notably the surrounding countryside is hilly. Also, the taxi dropping them off has an Oslo roof sign.