Hi There :-)
We have a solitary white bluebell down in our woods, amongst a mass of 1000's, having googled about them, an article from 2009 (link below also mentions the NHM) suggested they are very rare, however this year i've seen many more along country lanes in cornwall - could the relentless wet weather of last (and this year) be a cause?
PS. This is by far the best year we have had at Trenython for bluebells, but worryingly considerably less polinators!
I'm not sure weather has anything to do with the colour. If it's a genetic mutation, it is spontaneous. Its propagation within the population can be affected by the environment though.
We have white bluebells in the museum's Wildlife Garden, and I've seen some in woods in the SW.
You are right it's a native bluebell. You can occasionally find not entirely white plants - perhaps the result of crosses between normal and white flowered plants but more likely just individual mutants that have localised loss of the blue pigment. I've collected nicely blue-freckled plants in the past but they didn't retain the character in cultivation so there may be some environmental factors which are also influencing this. The most attractive I ever saw was found by an ex-nurseryman in Somerset and graded from white at the tips to typical bluish-purple at the base. It would be interesting to know if your plant does the same next year.