I'm sure they've been stocking up weapons at Fairford, Moron and Diego Garcia for quite a while now. I'm sure any BUFF crossing the NAT system came with at least a belly full of toys. When you stick them on the wings, drag goes way up. Requiring more air refuelings. It would be worth dragging them under wings if they were smarter, high value weapons like JDAMs or ALCMs. But for dumb stuff like MK82s, I'm sure there's plenty to be had all over England.
The plane you saw could have been an aborted mission for any number of reasons. With the emphasis on minimizing civilian injuries, great care is taken not to produce any errant bombs.