Richard J. wrote:

I believe that the arrows on the carriageway are only advisory.
However, is there a sign in advance of the junction showing the
allocation of lanes, like the one at
http://www.highwaycode.gov.uk/images/signs172.gif ? Someone told me
that these have to be obeyed, but I can't find anything in the
regulations that says so.

It seems unlikely that that particular one would have to be obeyed, since
these commonly appear on roads where the bus lane can also be used by taxis,
and it would seem unlikely that taxis would not be allowed to travel forward
in the left hand lane when they are allowed to use the bus lane.