It's Section 72 of the Road Traffic Regulation Act 1984 and its scope is limited to the following and only with the prior consent of the highways authority and subject to any conditions imposed by them:-
(a) a stopping place for public service vehicles;
(b) a warning of the existence of any danger; or
(c) the name of the parish or community or of any place in it
Apart from this, the only other power I am aware of relates to footpaths etc, for which Section 27 (5) of the Countryside Act 1968 states:-
"With the consent of the highway authority, any other person may erect and maintain signposts along a footpath, bridleway, restricted byway or byway."