Perhaps the more "correct" tag would be maxweightrating, however, before we (OptimoRoute*) started mapping in the UK we looked into how it's been done so far - and generally from what we saw mappers up until now used maxweight for the vast majority of signs, while reserving maxweightrating for bridges and the like (for signs which specifically point out maximum gross weight, for example). 
As such, we decided to opt for the more common tag both to respect the local customs, so to say, but also to perhaps even reduce confusion for less experienced mappers not familiar with the other tag. 

*If you want to perhaps read about our project, you can do so here: https://wiki.openstreetmap.org/wiki/Organised_Editing/Activities/OptimoRoute.
