They're gorgeous!
Cacti & succulents are okay with sub-zero temperatures for short periods but sustained, daytime, low temperatures with not much light might not be good?
Sunniest corner of the greenhouse would probably be your best bet but if we get a really chilly Winter a cool, bright spot indoors?

I would bring them indoors and put them in a bright sunny place if you can. They are much too attractive to hide away in the greenhouse, and if we get a bad winter a hard frost could well kill them. Cut back on watering. The cooler the room the less they will need - in a pretty cool room they can go a month without water in winter. I couldn't actually see any cacti- the succulents should be a bit more tolerant of over watering but under rather than over is the rule. Let the Echeveria go on flowering as long as it will before drying it off to much.

Some succulents can take colder temperatures than others. I can see a Kalanchoe tomentosa in the 2nd photo and that would prefer above zero.
However the Sempervivums will survive several degrees of frost as long as they are dry.
You would also have to ventilate the greenhouse well when it isn't raining … they can become damp in winter and succulents hate that when it's cold.

They're very pretty collections but I can't see any cacti there though Marjorie.

The flowering one is a Sempervivum … the flowering rosette will die after the flower fades but the offshoots will keep on growing.

Sorry Hywell you're right there isn't a cactus in sight and only have one.
Thanks Darren,Stergram and yourself for your excellent advice.