That Oxford example, at least, does have a "get in lane" sign[1] as you approach up the Woodstock Road, so any driver intending to go straight ahead on the A44 should know to use the right-hand lane to enter the roundabout.

But of course not everyone notices every sign (and some drivers just wilfully disobey them if they think it'll save a few seconds...)

[1] https://www.google.com/maps/@51.787062,-1.2794264,3a,15y,317...