Firstly, not really sure why these are called surface boxes. They don’t sit on a surface. Very much like a gas meter cavity box, this box will be attached to the side of your property.
Unlike the gas meter cavity box it will not be built into a cavity in the wall, but instead, just be attached to the wall. This does mean that the gas meter surface box sticks out a lot more than its cavity counterpart.
What is very different about the surface box, and is perhaps why it is called a surface box, is that it will be situated lower than a cavity box is. This does mean that it is not as easy to read the gas meter inside the box.
You will often see these boxes on old terraced houses, that were built before gas was common. These types of houses wouldn’t have a cavity space for a gas meter cavity box and fitting a semi-concealed meter box would be difficult as the house will usually have concrete next to the wall.
One issue with the surface box is that the doors do often break, and getting a replacement door is not as easy as it is with a cavity box, as very few places sell the doors on their own.