I don't think it is a legal requirement, but it ought to be.
For example in Italy every plot of land or dwelling seems to have a minimum sized white plaque with black numbers on at a height that can be read from a van vindow.
In this country people whose houses have both names & numbers should be made to give out the number as well as the name. Finding Dunroamin is a lot easier (and therefore more likely to be achieved) if you know it is between number 279 and 283 on a 3 mile stretch of road road
There are some villages spread out over 5 miles of roads in North Yorkshire where NONE of the houses have numbers, but the postman has spent time & effort in drawing out a plan of the village and fastened it up in the pub porch or bus shelter.