Its not meant to be anything in particular - the front of it looks very non descript, however look past the A pillar and it looks very much like an older Sierra, look briefly from the back near the end of the advert and you'll see that this is what the car is based on. The grill is not a standard Sierra grille. Looking at the ad in detail there isnt any cars that stand out to be definite models, just look alikes.
It was originally a sierra. It seems that for many ads they take old cars and then disguise them so they don't look like what they once did. This seems particularly true of some break down ads where the RAC or the like don't want to be accused of suggesting one manufacturers cars break down. Most of the floating cars in the mondeo ad seem to be old fords.