RRFSO = Regulatory Reform (Fire Safety) Order 2005 is the Statutory Instrument which started the whole process off.
Basically the Government put an Act through Parliment (in this case the Regulatory Reform Act 2001) which broadly gives the government power to do something. This is a long process a reading in the House of Commons and possible ammendments followed by a reading in the House of Lords (more ammendments), then to Committees back to the House of Commons for a second reading, House of Lords, Committees, third reading House of Commons , House of Lords then to the Queen for Royal Assent then it is LAW. The whole thing can take years, they can rush a law through in about 3-6 months if everyone agrees with it.
A Statutory Instrument is an Order made by a Minister or Secretary of State under powers given to him by the Act which doesn't need to go through the whole rigmorole of passing an Act, hence it can come into force a lot faster.
If you look at the government web site you will see that there are a lot of SIs compared to Acts. Some of the SIs are large like the RRFSO (or RRO) some are about a page long on a subject like raising the duty on petrol.
I would think there is a very similar process north of the border but maybe under different names.