Stewart Robert Hinsley writes

I am under the impression that the white and pink variants occur in
parallel in both Common and Spanish Bluebells. There are a number of
other characters that distinguish the two species -
URL:http://www.google.com/search?num=100&hl=en&q=English+Spanish+bluebell
will find you text on that subject, but that will not necessarily apply
to all hybrids - if you've got some introgression into a population of
Common Bluebells all you can do is rogue obvious hybrids as they occur.

I expect that pink and white variants occur more frequently in
populations grown from cultivated stock, rather than from wild stock,
as variants have been selected for horticultural use in the past.

I'm pretty sure that you get pink variants of english bluebells (and I
can remember seeing the occasional one back in the 50s in wild bluebell
populations when spanish bluebells were far less common in this
country), but I think that pink and white variants are far more common
amongst the Spanish bluebells. Whether this is true in the wild
population or whether it is, as you suggest, a result of cultivation and
selection, I don't know.

Plantlife has more information on its page, including an identification
chart and the report of their recent survey of bluebell populations -
worth a read, but only if you are on broadband - it has a lot of large
pictures!

http://www.plantlife.org.uk/
--
Kay