A. Move to the right
B. Slow down
C. Change gear
D. Keep to 30 mph
On minor roads with low traffic volumes it's often not financially viable to replace a bridge that was been built 100+ years ago to predominantly service horses and carts transporting hay.
These bridges are often sturdy, brick constructions and can be maintained simply by fixing the tarmac; to build a new bridge would require earthworks, road closures, diversions, compliance with new building standards, planning permission and more.
As you approach a narrow bridge be prepared to stop if necessary. If it has a humpback with solid sides then your view of what's on the bridge could be compromised.
If there are pedestrians or cyclists on the bridge they might not be able to get out of your way, especially if you are driving a wide vehicle.
As the bridge is narrower than the road, take care if you are towing a trailer that is wider than your vehicle as you will need to leave more gap on your left. For single-lane bridge, aim down the middle.
On humpback bridges you should be careful of grounding your vehicle if it's a long, low vehicle.
The image below is a humpback bridge in Sussex. You can see that when pulling out of this junction from the right you can't really see what's coming over the bridge as the height of the bridge and the bridge parapet block your view.