It's possible, but I don't think so.
My LF-2 chart for France also has 1 : 514 000, but near the scale it clearly says to use a 1.03 factor for distances.
The UK chart just has a scale with the Lambert projection parameters above, and that's it. I would assume their printing target is indeed 1 : 500 000 at the standard parallels, and guess that the variation is down to the variable water content of the paper, especially after printing.