It also changes during the life of the flight's selling cycle, which lasts 355 days on BA. The overbooking limit may be a much higher percentage at some points in time than at others.

Also, I suspect that it won't be a hard fixed percentage. The percentage will be different for each cabin. And because the no-show rates for (say) a no-changes no-refunds I class ticket will probably be significantly lower than for a fully-flexible J class ticket, the overbooking limit at any time may vary depending on the mix of tickets actually booked.

Finally, don't forget that BA often books a virtual aircraft configured differently from the real aircraft, with the intention that there should be op-ups on the day. So while the physical aircraft may be 14F/70J/30W/185Y, BA might actually take bookings on the basis of (say) 4F/40J/20W/235Y, and then overbook each cabin on that basis.

And although I have no inside information about any of this, my first guess would be that your figures are conservative for at least some points in the selling cycle, given that the typical no-show rate on the day is about 10%.