I thought the Snowdrop was a native British wildflower but it seems not.

This is from the publication Wonderland (by Brett Westwood and Stephen Moss):

Though they were once considered native, botanists now believe they were brought here from continental Europe to adorn Elizabethan gardens.
The first definite record in the wild dates from the 1770s, when they were discovered in Gloucestershire and Worcestershire. If these showy flowers were truly native before then, it is hard to imagine them being overlooked.

I’m now seeing plenty of Snowdrops when I’m out and about on my exercise walks, though I’m not sure whether they’re naturalised non-natives or have been planted along the roadsides by green-fingered locals. There are several different varieties of Snowdrop, and I’ve also seen quite a lot of double-flowered varieties amongst the more common types. The doubles (pictured on the right above) are probably Galanthus nivalis Flore Pleno, according to the identification crib sheet on the BSBI website, which, if you’re interested, also gives clear details of how to ID the single-flowered varieties.