I've dragged this back up because I had a better look on the way back down today. The signs were (for example):
The M6 is obviously the motorway, the 370.00 is the location on the motorway (agrees with the markers), but I don't know about the Q or the 2. All the ones I saw had the Q, some had the 1 or 2, and others had no final number. I think they all had a double slash in them. So, that's most of the sign deciphered... One was definitely a culvert, the other looked like an ordinary bridge (but was just to the side, so could've referred to something else), and a third was next to a grating.
All to do with SMIS (the HA's structures asset management system) and Structures Keys which are the HA's unique identifier for each structure.
I normally have a consultant to deal with this for me so IIRC the Q will be a type identifier e.g. buried culvert and the number will be the instance so there may be no number, 1, 2 or more structures of the same type at the same location.
Have a look here
for the information needed when a structure is constructed. This information is all kept in a record against the structure key number.