Simon, I'm no expert about British headset sizing but I have had similar experience with now two old British frames, both Claud Butlers, in that the head race size that Sutherlands says is British [30.0] are loose in the head tube so it was better to press in 30.2 head races which Sutherlands says are Italian. On both the crown race inner diameter is what Sutherlands says is British. So I assume there are two standards as Pete Paine says (he's very knowledgeable) or perhaps my head tubes were reamed sloppily by an intervening owners or enlarged by overzealous head race pressing or who knows what. I have other British frames where this hasn't been an issue.
I considered shimming with thin brass sheet stock but a solution I found was to assemble a mixed headset from loose stock of headsets. I've used a Brampton threaded head race, Campagnolo Gran Sport head tube races, and a Brampton crown race, all with a suitable British locknut. The Campy Gran Sport head tube races are smooth, unmarked, and rounded like some Brampton head tube races I have, so they look right. The Campy Gran Sport head races mate very well with the Brampton threaded head race and the Brampton crown race with the right size of gap. You don't want too big of a gap between the complementary races.
Campy Gran Sport headsets are available NOS at http://www.campyoldy.co.uk/
Peter ...................................... Peter Brueggeman La Jolla California USA pbrueggeman(AT)ucsd.edu
John Betmanis said "...According to Sutherland's 3rd Edition: English and JIS is 30.0mm; French and Italian is 30.2mm ... Could it be that someone machined down the 30.2mm parts to fit an English frame?..."
Simon Bird said "...two sizes of british headsets! i only found this out recently when a headset i was interested in was advertised on ebay as having a 30mm 'spigot'. the spigot turned out to be the parts of the bottom and top head races that fit into the head tube. as you all know british standard is 30.2mm. so what's up with this oddball size? it was explained to me by the vendor - peter paine, that the sizes could be told apart by the presence of a groove around said spigots on the 30.2mm size. trouble is both my headsets have grooves but the alatet which i wanted to use falls out of the frame- headset press not required. were these 30mm headsets common? would it be possible to have the ID of the head tube shimmed or built up with brass? ..."