I take it you've never been on an early business train out of Manchester or elsewhere!
The 0659 and 0714 from Milton Keynes to London Euston (don't know where they originate from) are without fail always at least 90% full in First on arrival into Euston.
Many peak time journeys heading out of Euston are also very full - Friday evenings in particular.
The reason for single seats on Voyagers is because the whole first class section is in a leading carriage. On MUs designed to travel over a certain speed, all seats within x metres of the front bulkhead have to be rearward facing. Check the seating plans out...
Peak trains that stop at Milton Keynes are
always full - in both First and Standard Class - because of those annoying Milton Keynes commuters.
In my experience, weekday departures between 17:00 and 18:59 from Euston are rarely more than 40% full in First Class. I don't know about the trains at or after 19:00, but from 19:40 onwards they are fairly quiet once again (except in Coach J). The 19:40 to Manchester was full in Coach J (the reservations coach), but had only 10 people in Coach G.
I didn't know about the Voyager thing though!

Even so, I find it hard to believe that Virgin never thought of inserting airline style seats (in turn creating single seats) to have extra First Class capacity. First Class would be overflowing on Super Voyagers if they had the same layout as Pendolinos!