Although you're not specific regarding the British boats involved, if you're talking about Explorers, Nordkapps and such, they are all much smaller in volume than your Chessie and much narrower. The lower volume reduces the effects of wind and the narrower beam reduces drag. These Brit' boats are also superbe rough water performers. On flat water, they're not known for exceptional speed, but when things get bumpy, they really come into their own.

Also consider your level of conditioning relative to the other paddlers. Personally, I find that my fitness level is generally the limiting factor in keeping up with others, rather than the boat I'm in.

It doesn't take much of a difference in either the boat or the "engine" to have a substantial effect on an 18 mile paddle.