Fortunately it doesn't happen very often. Hit a railway bridge and the charges are colossal - something of the order of £90 for every minute the railway line is shut in some cases.
There has, by law, to be a sticker in the cab of any HGV that is taller than 10 ft (3.05m). If the vehicle is an artic pulling trailers of different heights it is the driver's responsibility to know the height and set the sticker in the cab to the correct one.
It could be argued that a company that doesn't supply these stickers is also at fault in the event of a bridge strike, but it is always down to the driver in the first instance.
The only way a driver could be found not guilty after hitting a bridge is if he can show that the height displayed on the signs was wrong. This has happened when a road has been resurfaced and not re-measured.