The gov.uk website does not mention driving to take your exercise - at least I cannot find anything about it.
Hmm. I can only apologise. I heard a statement where they said it was OK and police had been over the top, but the gov website doesn't say that. In fact I can only find advice against it. It seems they're giving discretionary powers to the police, which is a bit worrying I think. There's certain things which are not enforceable by law yet.
Anyway sorry, bluebells! The good thing is they'll be there next year if you miss them. Just remember that.
My local bluebell woods is ashridge near Berkhamsted.