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I'd consider running 38-40
I won't go that big. I have that size on my touring bike which I have basically used as a mountain bike, ridden up mountain trails in Albania, down river beds in Wales, etc. But they are slower on the road. I don't think the terrain this year will be that bad - it's just 100km of gravel or whatever out of 4000km which will mostly be better than UK roads.
Tbh I'd consider running 38-40. You wont loose anything aerodynamically and perhaps gain a lot in the gravel. Also comfort!