Why is it so poorly rated?

Perhaps because the film is complete drivel? What a waste of talent! It's a shame when older actors resort to doing films like this in order to keep working.

Possibly because the script was made as lame as it gets.

Just imagine... at the cliff, after Manon rescues them from the van, its weight could be just too much for her little car's parking brakes to hold, and both empty vehicles would fall down to the drink. All three would be stranded there. So Kate would call Jean-Baptiste with his boat. Yes, they would immediately realize the age difference as shown, but Manon and Jean could have had love at first sight right there.

Several other lost opportunities there.

Because it clearly doesn't work very well. Not sure what style they were after but the whole thing felt clunky and the music was rubbish. The only plus point is Emma Thompson looked lovely.

i think it's good fun, but I didn't think it was brilliant. enjoyable though.