I must admit, when BA crew report for flights we get a briefing sheet which indicated the booked load...very very rarely do I notice an over booking in First. Occassionally when a 13 F seater is subsituted for a 14 F seater (On the B777) you will have a single over booking, otherwise its usually only on the East Coast high frequency routes such as JFK/EWR/BOS/ORD etc which you will find a single overbooking...simply because of the high no show rate.
Now this by no means makes it acceptable but it would appear that BA never OB more than 1 Pax in First. They do however seem to OB about 5% in Club on occassion but this is not an accurate figure!