Yes, there's a good shop between Sheffield and Chesterfield, called 'Steels' which sells a good range of varnishes. They don't do mail order but as you can't find what you want in London I'm sure you won't mind a bit of a drive.
Or failing that, take your boat upstream to a town called "Shepperton" (about 5 miles upstream of Hampton Court) where you will find a shop called Nauticalia, right on the Thames.
They certainly sold it a couple of years ago, might be worth ringing them first though.
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