If the pics are the ones you don't know, the top one seems to be Phlox paniculata, the middle one is a Regal Pelargonium and the bottom one I think is Scabious. The first and the last one are hardy, but the Regal Pelargonium is not, along with the petunias you mention.

Fuchsia: some are hardy varieties, some are not, but all fuchsias are vulnerable to frost when kept in pots outdoors, so best to consider them all tender, and overwinter inside, especially if you don't know the varietal names.

Osteospermum - there is a hardy one, but most of the varieties sold are not hardy and will need protection (greenhouse or conservatory) in winter.

Geranium - I don't know whether you mean Geranium (cranesbills) or whether you're using the common name geranium meaning pelargoniums. If its the latter, these will tolerate cold temperatures down to -10 for short periods, BUT, they will die because of the damp and cold combination in the UK, so most people overwinter them in a greenhouse or indoors.

If we have a very cold winter, all potted plants are vulnerable; if the temperature stays below zero day and night for a week or more, the soil in the pot freezes and kills the plants. This circumstance is pretty rare though, we don't usually have those sort of winters. Its safest to move hardy plants in pots together, preferably against a house wall or something where they have more shelter, and in sunnier spots.

I think the first is a sweet william, Dianthus barbatus, not a Phlox. It's biennial so won't flower next year (it does sometimes hang on, but always looks straggly to me...). The Scabious should survive but I haven't had much luck with the dwarf ones.

You may be right about the top one, Landgirl - depends on height and overall growth habit, and when that pic was taken...