The five reported users of the LONDON mark are:
Middlesbrough Pottery, 1834–52
Isaac and Thomas Bell, Albion Pottery, 1860–63
Carr & Sons, Low Lights Pottery N. Shields, 1844–c 1900
Malkin, Walker and Hulse, British Anchor Pottery, Longton, 1858–64
Fell & Co, St Peters Pottery, Newcastle on Tyne, 1817–1890
So we are back to where we started, with a plaque, most probably John Carr & Co, stamped LONDON for distribution there. As always, if anyone else has an item of lustreware marked LONDON, please share it.
P.S. I think this explains the mystery mark on a Carr-type plaque, mentioned in my 30 May 2010 blog post. Look at the mark above. Isn't it just possible that a cataloguer might have mistaken an indistinct impress, particularly if viewed upside down, for 'MOORE'?