In England and Wales, it is illegal to smoke anywhere on the station, whether it is inside a building, or on an open platform.
In Scotland, the same applies, expect that it is LEGAL to smoke on the platform if is not covered.
With the exception of the two Network Rail managed stations; Edinburgh Waverley and Glasgow Central where the enclosed areas are covered under the Smoking, Health and Social Care (Scotland) Act 2005 and open areas under Railway Byelaw 3, the signs displaying the appropriate legislation for different parts of the stations.