simonk
I would be very surprised if you can find any legitimate source for actual number of seats available. I would be surprised even if travel agents have access to anything other than the number of seats available for sale by class - ie including the overbooking profile. The reason for this is that it is commercially sensitive information that could be used by others. If you are a travelling on a commercial ticket with an OK status just turn up at the appointed time, if on a ID ticket with another airline you will have to take your chances or visit a travel agent to see what they can tell you or rely on a contact within BA.