A friend is about to built a hybrid of all three elements of a
Greenhouse/Glasshouse/Conservatory.

Is Horticultural glass still used for anything or has H&S banned it?

just taken delivery of a greenhouse where standard spec was horticultural
glass (I had opted for tempered glass instead for safety reasons)

I have always assumed that horticultural glass was simply a cheaper form of
glass than domestic. It will allow the same light through but does not need
to be so optically perfect, ie glass with some optical flaws - seconds. Also
probably thinner - 3 mm. If you have big panes of glass and want to admire
the view, you will want them to be thick and optically perfect.

Tempered glass is more expensive and is used for safety purposed, it depends
on whether your intention is to look through the glass or walk through it!
If you have children you probably want tempered glass, but I do not see that
it is neccessary for adults.