Timings will also vary between 1 and 2 tickets, as if you're on 2 tickets changing from Virgin Atlantic to BA, then BA are likely to tell you to buy a new ticket if Virgin get you into Heathrow too late for the onward flight. Unless your travel insurance will cover that, you'd either want to allow plenty of time in case of delays, or book the USA-UK leg with BA or AA so you'll have more protections in case of delay on the first leg