One Post Office branch obviously doesn't want to keep special stamps cluttering up the safe for too long so they placed the David Bowie stamps on sale last week, possibly at the same time as the Windsor Castle stamps (or instead of?).
A correspondent has sent this picture of the Hunky Dory stamp with a Jubilee Mail Centre postmark of 17 February, although in this picture it looks a bit like 17-03-2017. The letter was sent from Sunbury-on-Thames (which doesn't necessarily indicate where the stamp was bought).
At the time of writing this is the Earliest Known Use of a 2017 Bowie stamp.