The only other thing I can think of is to bring the glass all the way
down to near the floor but I don't particularly want to do that as I
don't think it would look as good.

I'd go that method. Full height glazed conservatories can look very good,
especially if rather than using large glazed panels that make it look like a
shop, you just replace a wooden panel with a small glazed unit or two.
Companies such as Baltic Pine would be delighted to supply you with wooden
frames and glazed panels in this configuration. They look particularly
handsome with glazed fanlights with a single Georgian bar such as the
following diagram, with all panels glazed. Personally, I think dwarf wall
conservatories look a bit odd and prefer full height designs, whether the
bottom panels are glazed or wood panelled.

+--+--+
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I've no idea what the regulations are trying to achieve, they seem
pretty crazy to me as the room is unheated and is outside the main house
walls etc. so there will be no heat loss through it.

The problem is that there is no way of preventing you plugging in a fan
heater and making it heated.

I'd use double glazing anyway. It isn't that much more money and will make
the room much more comfortable, particularly due to the large glazed area.
Also, consider using triple wall polycarbonate (or better still, double
glazed roof) for the same reason.

The building regulations don't require you to use cavity wall methods. Just
bolting on sufficient thickness of Celotex insulation is enough and is
recommended anyway. However, due to the large glazing requirement, you are
unlikely to be able to comply with building regulations without using the
conservatory exemption.

Christian.