Gaza,
I'm glad you got bag in time, whilst clearly not ideal, I'm pleased to see they got your bag to you quickly, it does save trekking around the Old Trafford Centre
. Glad it cleared things up, I know as a passenger you can sometimes think "how is this possible?" but when the holes in the swiss cheese line up unusual issues can occur. On occasion only with a long explanation can fully explain what's happening and why. For most passengers this long explanation would be baffling, therefore the simple reason is normally given, often followed by passenger's pulling funny face's and thinking its all "rather ridiculous and that BA wouldn't have this problem".
Ph1l1pncl
As WingoWango mentioned taking off the extra fuel would unfortunately defeat the object of carrying the extra fuel in case of diversion. If for example they were unable to land in MAN and needed to divert, but didn't have sufficient fuel, the situation would be a whole lot worse, dire in fact. As an example a few weeks back our diversion fuel was considerably greater than our trip fuel (fuel that would be burned between our original departure and arrival airport) due to the fact that most of the UK had fog forecast for the best part of the day. So hopefully you can see that a route that may only burn 800kg's of fuel en route may actually require something close to 2500/3000kg onboard, if the weather or other factors dictate.
As others have also brought up, the question of mentioning the bags is difficult. It can lead to outrage amongst the passengers, demanding whose bag will make it, whose won't and why should the person next to them get their bag and not them? Consuming more time than if some were taken off and put on the next flight two hours later. It's a tough call to honest.
