Happens elsewhere too. Amex now has a feature on their online account platform where you can see "pending charges" in addition to those processed. Quite often, you'll see duplicate charges for vendors ranging from gas stations to restaurants to Amazon.com. I've also had this happen on other card accounts from various airlines (a recent Emirates purchase comes to mind) so this does not appear to be unique to BA.

A previous poster's point about whether such holds effectively reduce one's credit limit dollar for dollar, albeit temporarily, is an interesting one.