Thank you for the replies. Quick update on how this was resolved in case it helps others.

We've ultimately rerouted so we're now LHR-JFK on the outbound and getting down to LHR from MAN under our own steam - accepting the connection risk.

VS confirmed that having a BA sector MAN-LHR in the Virgin revenue booking was a valid routing for the cancellation rebook. However, couldn't ultimately do it as it wasn't valid on the companion award booking as, a) the companion needed to be travelling on an identical itinerary, b) the companion could only be on VS metal. A Sky Team routing via AMS or CDG was also possible on the revenue ticket, but not on the companion booking (even if there was reward availability).. The companion could be booked into any fare bucket irrespective of award availability, but had to be on VS metal - as BA do in the event of cancellations.

I've selected a departure with VS's new A330-900neo - fingers crossed for no equipment change.

Another quick follow-up question:

I’m now arriving at T5 on a BA J ticket, and then flying out of T3 on VS. I'll obviously be able to use the VS Clubhouse, but will my arriving BA J BP afford me the use of the OW lounges in T3 too?