Thanks for replying. There are a lot of mis-taggings for access in OSM, so it's not catastrophic, but hopefully we all want mapping to be as accurate as possible and hope that renderers and routers catch up.

What I know about actually implementing routing services would look lonely on the back of a postage stamp, so take this with a grain of salt ...

Ideally a more specific access tag should override a less specific one. For the case of buses, bus=* should take precedence over psv=*, which should in turn take precedence over motor_vehicle=*, vehicle=* and finally access=*

If you have access to street side imagery of the place you're checking (with an OSM compatible licence), there's a guide to interpreting UK traffic signs on the wiki.
https://wiki.openstreetmap.org/wiki/Road_signs_in_the_United_Kingdom