OS Locator is useful. As I have said before it helps OSM mappers in Great Britain find places where the names of roads we have mapped disagree with the names Ordnance Survey have. The reasons the names differ are many and vary across different types of road. Many country roads just don't have a name board, so if we use the OS name there can be no difference - except that the names on OS StreetView are sometimes different from the names on OS Locator. This might imply more errors being possible where all the roads are named. There are typographical differences such as names with or without a space (Tweendykes Lane vs Tween Dykes Lane) or use of apostrophes. Sometimes the name is just completely wrong. Just occasionally a road can have two or more name boards that do not agree, but I've only seen a small difference such as use or not of an apostrophe.
OS do not provide the definitive name, that is the local council that the road falls into. Where a road straddles two authorities this can cause a problem: a road called Endyke Lane in East Yorkshire turns into Endike Lane as it crosses into Hull, though now not exactly on the boundary.
I believe the name board on the ground to usually be correct, after all if the board was wrong people on that street are likely to ask questions of their council and it may well get changed.
I saw a comment earlier today on twitter from Tom Chance (@tom_chance) about the discrepancies in Southwark and they looked to be at a similar level to East Yorkshire and Hull that I have worked on. I thought I'd take a look, and ended up examining some some from the very useful ITOWorld OSM Analysis page. I took the twenty nine areas that were 100% complete and worked out the percentage of name anomalies for each area. The graph of the results is below:
This looks like a spread of differences that might reflect the fact that different surveyor work in different areas and everyone makes mistakes. Has an OS surveyor misread or misrecorded or mistranscribed a name? I feel that many of the anomalies will have been rechecked on the ground by OSM surveyors, in case they were wrong, so in general I trust OSM more than OS. I hope that no one just uses the OS names in the belief they are always right. There is also a very small chance that both OS and OSM have misrecorded a name in the same way so they agree (no anomaly) but are both wrong.