Transition periods, they're always a sod. Seven SSNs is all we're getting though, even when the Astutes are all online and the Traffies withdrawn we're only going to have seven Astutes, we only had seven Traffies, but we also had the Swiftsures running down at the same time (and there was only ever six of those). Our SSN fleet hasn't really gone above double figures in decades. We just don't have the manpower or money for it, and I can't see
that changing post-Brexit.
Pretty unusual for
all of them to be offline at the same time though, I mean they're not that old, no older than the Los Angeles class. Must be that the problem lies more with maintenance costs and scheduling.
Not too surprised that Astute might be offline though, she's been a bit of a problem boat since her launch, and Ambush ambushed a merchant with its conning tower last year

, so I imagine if one boat is active it'll probably be Artful.
EDIT: According to the Beeb just now, Astute is the one who is at sea after maintenance so Artful must be in dock for repairs for some reason.