Are low bridge signs the height of the bridge or the maximum height of the vehicle that can fit under?
Both,with maybe a little leeway but don’t bank on that.
The sign shows the clearance, or maximum height that will fit under the bridge.
So if your height matches the bridge, you can fit under it. In theory!
It is also breaking the law to take a taller vehicle under it, even if it fits.
Santa:
It is also breaking the law to take a taller vehicle under it, even if it fits.
That depends…
On the signage.
A white cirlce with a red ring makes it an offence to exceed the stated hight.
The maximum safe headroom at an arch bridge is shown on the triangular warning signs. If you can pass under it without hitting it, you haven’t commited an offence.
I always worked on the principle if a bridge signs says a certain height ie 14.00 thats the height of the bridge, you need to give yourself ample clearance.
Another thing is bridge heights don`t always account for the length of a vehicle or any rise in the road before you vehicle is all the way through.
For example you might get through a bridge with say a 33ft,but hit the back end with a 40
Given that a red circle is a prohibition sign, I’ve always taken the number in the middle to be the start of the restriction. For example, you couldn’t take a 14"10’ trailer through sign saying 14" 10’, but you could a 14"9’.
Captain Caveman 76:
Given that a red circle is a prohibition sign, I’ve always taken the number in the middle to be the start of the restriction. For example, you couldn’t take a 14"10’ trailer through sign saying 14" 10’, but you could a 14"9’.
Do you do 29mph in a 30 zone for the same reason? 
slowlane:
Captain Caveman 76:
Given that a red circle is a prohibition sign, I’ve always taken the number in the middle to be the start of the restriction. For example, you couldn’t take a 14"10’ trailer through sign saying 14" 10’, but you could a 14"9’.Do you do 29mph in a 30 zone for the same reason?
Yep. 
And everyone appreciates me doing so. They all flash their lights and wave at me.
My take has always been; if a sign says 12’6" and your lorry is KNOWN to be that height or less, if it hits the bridge it’s their problem.
If on the other hand the sign says 12’3" and you hit it with that 12’6" lorry, it’s your problem.
It’s worked for me for 35 years.