How cold is ’pretty cold’?

I don't think it's much warmer than outside really - there are no radiators in there. I imagine it will be down to a couple of degrees centigrade on a winter night.

Sorry, I'm in the UK.

Not cold enough to be interesting like Canada, just cold enough to be a bit rubbish :-)

Down to a couple of degrees centigrade at night in winter.

Yuccas and the fatsia can take a lot of cold....those you could very likely grow outside without concern so a cool, even cold conservatory should be fine.

As long as it never really drops below freezing, a lot of marginally hardy plants would work - bananas, some ficus, hardy schefflera, palms, etc.

OK. Now we know where you are... I have a small unheated conservatory in the UK with a large Hoya all around the walls. Fatsia would be fine but it would also be perfectly happy outdoors so inside space would be a bit wasted on it. Unless you live in the North or somewhere high up the same goes for Camellia, Yucca, Euonymus japonica (very common and practically a weed here), Cordyline, Loquat, Podocarpus, and Trachycarpus. Dicksonia is a bit marginal but is ok outdoors in sheltered gardens in many places And there’s nothing to see in winter. I’d be thinking about Citrus, Ficus, Norfolk Island Pine, Palms or Schefflera. A Jasminum polyanthum trained on an obelisk would give you winter/spring blooms, scent and evergreen leaves. There is a list of shrubs for a cool but frost free conservatory here https://www.rhs.org.uk/plants/types/houseplants/cool-conservatory-greenhouse

'Citrus are sun and HEAT lovers., so too with the Yucca.'

That's a bit overstated :-) Some citrus can grow outside here but most remain indoors for most if not all of the year and so never get much heat or sun, yet manage to thrive AND produce fruit! Same with yucca. Some varieties are hardy down to zone 3 so easily tolerant of below freezing temperatures and also very adaptable to less than full sun conditions. There are also many hardy or borderline hardy palms that would thrive in these conditions.

There seems to be some misapprehension that a north facing conservatory will somehow not provide adequate lighting. Since a conservatory typically includes 2 or more glass walls AND a glass roof, there is generally plenty of light available. Unless there is some obstacle or structure nearby shading it. And since conservatories are attached to heated structures, there is generally plenty of ambient radiated heat to keep things comfortable even in the dead of winter.

We don’t know where exactly the OP is situated but where I am the average winter temperature is well above freezing. February is our coldest month and the average is 40.5f

I kept a Meyer lemon for many years in my little conservatory which is shaded by the house for much of the winter.

@floral_uk z.8/9 SW UK and @J BUCK….hope you and your conservatories were safe from the terribke UK storms.

Thanks. Mine is fine. But I did get stuck on a train yesterday which could not go any further because of a tree across the line. We were supposed to be on our way to London. In the end the whole network shut down and all the London stations closed. Luckily we had been held at a station, not in the middle of nowhere, so we just got a taxi home ...... by around about route because of trees across various roads.