Saying "the London Fancy is a Lizard" is like saying a dog is a wolf. They may share a common ancestor, yet they are fundamentally different — and in the London Fancy's case the difference is genetic, not merely visual.
Shared origin is not the same breed
Historically, the London Fancy most likely arose within Lizard populations in 18th-century London, among the same community of breeders in Spitalfields. That shared origin is not in question. But a shared beginning does not make two breeds one.
The genetic difference
The London Fancy phenotype is caused by a heritable mutation called Progressive Greying: a progressive loss of melanin-producing cells after the juvenile plumage. This is not the same mechanism that produces spangling in the Lizard. They are two entirely different things — as documented by Hein van Grouw, one of the world's leading authorities on colour aberrations in birds.
The proof from the breeding room
Across a stud of 40 London Fancy pairs, over several seasons, not once has a single Lizard-like throwback appeared — no spangling, no rowings. If they were the same breed, such throwbacks would be expected. They do not appear because these are distinct breeds.
Why you should not cross the two
Crossing London Fancy with Lizard destroys what defines the breed: it splashes melanin across the body, messes up the wing patterns and removes the clean contrast. A single pairing can undo years of selection.
Go deeper: The London Fancy Manifesto, with scientific sources.