Actually neither, you should use the international boundary line for this. Alternatively, if you were going by ferry, then ships are free to keep whatever TZ they wish, though the guidance is the 7.5 degree zones with no summer time/DST, and the usual bumps around the date line.
Also, when you're on going through the channel tunnel do you go from being closer to Paris than you are to London only when you've left Calais? Or before you get to Dover?
By rail the distance is 491.5 km. The tunnel portals are 162.05 (France) and 111.60 (UK) km from London. The 245.75 point is just west of the Erquinghem crossovers near Armentieres, about 21.4km west of Lille station. Tim Smartcom Software Ltd Portsmouth Technopole Kingston Crescent Portsmouth PO2 8FA United Kingdom www.smartcomsoftware.com Smartcom Software is a limited company registered in England and Wales, registered number 05641521.