If the 1978 Scotland Act had passed and the assembly created it would have certainly become a focus of resistance to Thatcherism in the manner of the Metropolitan County Councils and the GLC. This means that in the mid 1980s 1 of 2 things would probably have happened:
1. It would have been abolished along with the GLC, creating EVEN MORE anti-Tory sentiment in Scotland, even sooner, leading to a total Scottish Conservative melt down by 1987 or 1992 rather than 1997. This would also feed Scottish Nationalism, the SNP would say that Scotland cannot trust Westminster, and that Unionism means being under control of the unaccountable London government that feels free to ignore the wishes of the Scottish people and go back on it's word. Maybe Scotland quits the union before 2011.
2. Thatcher doesn't quite dare to abolish something with strong public support in Scotland on top of the other changes she is inflicting - she backs off. It remains in place as the only large, independent local government structure, and is able to preserve public services in Scotland. People in the North of England are jealous of this, and when Labour comes back in 1997 similar Assemblies are created in Wales, the former Metropolitan counties and Greater London. This is followed by a system of devolution taking in "Shire" England, and the UK evolves into a federal nation similar to Australia or Germany. A smaller house of Commons and a UK Senate are the new federal parliament with distinct Union level powers and responsibility, and the internal contradictions of Asymmetrical Devolution - the West Lothian question - don't arise.