It just doesn't look anything like any of the London Witcombs I've seen in photos. They don't seem to have used these lugs. Peter Weigel looked at the photos and confirmed it's not a Witcomb USA, but could not say whether it might be an English one.

I recently contacted the man who keeps a registry of MKM bikes; the serial number on mine matches their format. But the location and manner of stamping it is different and, again, it appears MKM didn't use these lugs.

The closest match I've seen so far was an early 60's Claud Butler (made by Holdsworth) which seems to have the same lugs and similar ends. But the serial number is all wrong for that.