I presume this is for a shed or something? It would be illegal to install
in
a house, except in some specific cases.

No, its not.

Provided the insulation is up to scratch.

That was one of the specific cases I had in mind. However, that's a lot of
calculations. You would have to ensure that the heating efficiency and
insulation of the entire house exceeded modern specifications to the extent
that there is some slack to allow single glazing. Given that the house
probably doesn't have such a specification unless built in the last year or
two, you would then need to rectify this. (Not a bad thing, in itself, but a
little excessive to just install a window).

It would almost certainly be simpler and cheaper to install a BS double
glazed window, which can look fantastic, as my parent's did with their
sashes. You have to look very closely to see that they are double glazed.

Note that no conservatory OUGHT to be allowed under he rules at all, -
they only slip through on teh 'elemental method'

They do no such thing. The elemental method requires 25% glazing. They slip
through on a special exemption, provided some rules are followed such as
independent heating controls (preferably no heating at all) and external
grade separation from the house (i.e. they must be separated from the house
such that they are considered exterior for the house calculations including,
normally, double glazed windows and wall insulation, depending on the method
used).

Christian.