Big caveat in that like
@ColinJ I think it's 50/50 at best on whether it goes ahead or not. However if we assumed that it was a normal virus-free year, then there shouldn't be any problem in signing up as late as the week before.
I've ridden it three of the last four years (ironically not this year though, but for scheduling reasons) and am pretty sure I remember tweets from the organiser in the days before each time, saying that entries were still open and "last few places remaining" etc. for the Sportive. The entries for the Race, GranFondo and TT do sell out beforehand, so if it was those you were after, then that is something you can't wait on, but the mass Sportive should be available right up until the day.