Plug Plants - where to over winter?

Greenhouse will work but not if it gets frosty in it unless you use a small heater.

Whereabouts are you in the UK, it doesn't say on your profile ?
Im thinking that maybe you could do as i did.

I'm in the south west and after repotting them (immediately) i placed the perennial plugs on my back patio and they were fine.
I planted them out in the spring.

Don't buy any that are too tender ie Bizzy Lizzies need heat. Pelargoniums and fuchsias overwinter in a cool greenhouse very well. What species were you tempted by?

I'm in the midlands in a moorland area so not particularly balmy weather!

I'm tempted by lots but really it's dianthus that I've set my heart on.

I've already potted on some lavender plugs and they are currently on the patio awaiting winter accommodation. Only 1 of the original 84 is looking sorry for itself but the weather has just started to turn so. I want to create a lavender hedge and plugs is the only way I can afford the number that I need.

Take cuttings as soon as possible, there's still time to do this. Dianthus should also be OK to overwinter. They like a well-drained soil with lots of sunshine, but shelter them from cold winds. The same goes for the lavender.

i overwinter dianthus cuttings in a cold frame with lid propped open for ventilation but free from wet/frost/snow etc.

That's right, it is often the damp that kills things not the cold. That's why we have Alpine houses with all the windows open.