I know it's not great news to be diverted - happened to me twice, the second time meaning I had an enforced night away with no spare clothes, toothbrush etc.
BA were in a no win situation - people will complain if they bus them to GIB, but might be complaining more if they sat and waited for hours in the hope that they could fly there later in the day, and couldn't!
I think they probably made the right decision at the end of the day to bus, as that way you were going to get to GIB.
If the bus left so late after you landed were you not able to purchase refreshments before boarding the bus?