I know that in England the act that banned smoking in public buildings was banned under the Health Act. 2006 but I am not sure what the bylaw is that bans it on open platforms.
I would say its a good idea to use the same rules as England for Scotland as it just brings consistency to the network. Northern Ireland railways have a smoking ban that is the same as National Rail in banning smoking on all railway premises.
Its strange though how at one point you could literally smoke almost anywhere except no smoking coaches and then the policy gradually changed to by default you couldn't smoke unless told otherwise, at one point as well you could walk round Tesco smoking.
hock:
From what I gather all local services and First group operated services had banned smoking by the time of privatization. Gradually after that smoking was banned on other services such as Virgin Trains (2001), Midland Mainline (2003), South West Trains (2004) and finally GNER & Caledonian Sleeper (both 2005). From what I gather all trains built after privatization such as the Class 170s, Voyagers and Pendolinos have always been smoke free regardless of if the operator had a smoking on some trains at the time. How so much has changed in the past 20 years.