These have been appearing in our garden in growing numbers for several years. I counted over a hundred this morning- mostly in a clump round one particular tree, but with several outliers. Richard Mabey calls them "one of the most local of well-known British flowers." Modern agriculture has driven them out of many of their former haunts but they can still be found in the wild in various odd places- most notably in water meadows along the Thames Valley. I don't know whether our snakeheads can be called wild or not; on the one hand they're growing in a garden but on the other we didn't knowingly put them there.
Mabey quotes the 16th century botanist Gerard as glossing the genus name fritillaria as meaning "of the table or boord upon which men plaie at chesse, which square checkers the flower doth very much resemble." Experts are divided as to whether they're a native wild flower or a garden escapee.