Flybe Embraer 195's might work.
This would be interesting to see and ties in well with the comments coming from Flybe that they'd like to explore contract flying opportunities with other carriers.
With crew bases at all the likely places that Virgin would want to be flying to LHR from, this could also be a lower cost option removing hotel costs for crew which might offset the reduced seating capacity.
With aircraft on order from Embraer, maybe Flybe would be able to free up some E195's to assist Virgin? It's an interesting prospect but would the partial BA ownership and codeshares be put at risk by such a move?