There's a narrower London alley than Brydges Place (London's shortest streets post, a couple of days ago). Phillip Barnett emailed me with the exciting news that Emerald Court (right), off Theobald's Road in WC1, is narrower.
I dropped in on both alleys this crisp, sunny morning, equipped with a tape measure. And I can confirm that Brydges Place is 33 inches wide at its narrowest point, funnelling you into St Martin's Lane at the side of English National Opera's Coliseum.
Emerald Court, however, squeezes down to just 26.5 inches at its north end, which squirts you into Rugby St. It's barely wide enough for my handlebars.
It starts at its south end as Emerald St, a promising-looking turn left north off Theobald's Road. It's quite possible you'd find yourself cycling up here, if you'd just missed your turn left up Lamb's Conduit St. It's also quite possible that you'd naturally keep going north, through the (pedestrian) alley which it narrows into, the Ozly-named Emerald Court.
So (unlike Brydges Place, which offers no plausible reason to go through with your bike) Emerald Court is currently our top candidate for the narrowest alley in London you'd reasonably go through with a bike. Unless, of course, you know different...
Thanks Phillip; I'll update the previous post, by the way, and add Emerald Place to the Google map.
Between the two I also checked out Lazenby Court, off Rose St, in Covent Garden. It's another very narrow alley, perhaps 36 inches at its narrowest, and it was very busy with commuters shuffling into single file. One of them had a Brompton; nice to have a bike that you can shrink down if the going gets narrow.