It certainly has or had more defoamer, I used to have a hell of a problem refilling the Octavia 1 and 2 with diesel in the UK, no problem at all in France, the Yeti has the problem licked so I don't know if the UK fuel remains as was or has now been improved.
You could be right regarding the calorific value, I always get less to the gallon in the UK but put it down to the journey profiles and traffic, since covid all my journeys aside from country to country have been local, last time in the UK my fuel economy did not drop, because of the fuel shortage I had refilled at Calais and drove for the 2 or 3 weeks in the UK on the same fuel, usually within a week I will have filled with UK fuel.
And now I come to think of it the journeys France to England (190 miles) vs the exact same return trip are always more economical and yet there is only 9 meters of elevation difference between the two properties, the fuel for each trip 99% of the time has been bought from the destination country.
Elevation does make a difference, I frequently travel 25 miles to Arras which is 40m lower than my property, there is an 6mpg difference between the outward and return journeys on the same roads at the same speeds, the mean of the two equates to my average for similar journeys.
Returning to sea level from the Alpes would give better economy than in the other direction although the difference would be minimised by the long journey, add in different fuel and it could be significant.