That is not normal and needs investigating immediately.
There is an engine mounting down there somewhere, often called a 'dogbone' as is shape resembles such. Its job is to restrict the engine rotational movment, especailly when moving away. One end fixes to the engine, the other to the car floor or subframe, presumably in the case of the Mk2, the floor. It's mounting point is sufficiently strong to take the normal forces. If the floor is moving, any amount, this suggests the floor is severely weakened. Most likely cause is corrosion, but could be collision damage, although this is unlikely as the rest of the front of the car should show such evidence.
You should not drive this, at all, until the floor is inspected properly. This could, sadly, be terminal, but hopefully, only for the car, not you.
If bought from a dealer, they have a legal responsibility to ensure it is fit for use on the road. If bought privately, no comeback.
How old is the current MOT? An MOT is only valid the day it is issued, but if the floor is corroded such that it is that flexible, it didn't happen overnight. A recent MOT may bring some comeback against the MOT testing station.
Good luck, but meanwhile, don't drive it. Sorry to be the bearer of bad news, but well done for asking.