There are two options.
( I am not familiar with Virgin ? superhub )
1. VoIP router with 'lifeline' facility (defaults to landline when no BB available).
Zyxel did some, in fact I have one, but the ethernet ports don't work due to power surge).
I believe that some of the current Draytek routers also have this,
look up Draytek 2110VG or 2920VG.
2. Siemens Gigabit DECT phone, which links to both VoIP (2 lines) and landline via Dect.
I think you can set which line is dialled by default.
You can see details on sipgate.co.uk website / shop.
Alas it' not cheap....