Snuffy, I think the only realistic thing is to go for the centre of gravity of the land masses taken together, assuming they are of equal thickness. That is how they do the "centre of England type calculation". Anything else would give far to much weight to Rockall and Muckle Flugga and bits of empty sea or whatever. I think it reasonable to reject the Channel Islands, as that is a bit of a historical anomaly, they are geographically French. But is beyond my feeble brain to work out how to stick the bits of cardboard together for the experiment, you need weightless rods and there aint no such animal.