I would describe many British lightweights of the 1930s not as having frames with extremely long top tubes, but as having frames with short seat tubes. Contemporary photo evidence from catalogues and touring photos, such as appears in the journals of the Veteran-Cycle Club, confirms this, for here we see significantly more seatpin showing than on bikes of the 1940s-1960s, whereas stem lengths appear to be about the same when similar handlebars are used, and the riders depicted are not unusually stretched out.
Contemporary Rivendells and Herons are not classic British lightweights, and their geometries are very different. Recent Mercians tend to be quite different from those built in the 1950s (my samples of that marque date from 1956, 1959, 1974, and 2001). I think most people who own and ride British bikes from the 1930s through at least the 1960s, corresponding to the vintage of many of the French bicycles Jan has referenced, would have to disagree with the statement that high-trail geometries are typical of British lightweights.
Christopher Barbour
Boston, Mass.