In UK law, a "statutory instrument" is a particular class of legislation defined in section 1 of the Statutory Instruments Act 1946. This envisages broadly three forms: Orders in Council made by the Crown; rules made by ministers where the power conferred is exercisable by "statutory instrument"; similarly for Welsh ministers.
In other words, most statutory instruments are statutory instruments because the legislation conferring powers on a minister to make them say that they are. There's no default. It is possible to confer powers on a minister which are exercisable without being an SI (though I cannot think of any examples of this).
I believe that there are some statutory powers to make SI's conferred outside the 1946 Act to parts of local government.
As I understand it, an Ordonnance (a French word) refers to a form of French legislation of a specific kind, whereas almost anything could be done by SI. Some SI's might amend Acts of Parliament; others might directly implement EU legislation; others might simply set speed limits or tolls on the Humber Bridge.
The point here is that SI's are a means of numbering and indexing legislation. That is all. An SI is not a particular class of rule having a particular status. The consequences of being an SI are essentially being given a number.
So I think it would be confusing to conflate the two ideas.