Generally I would agree with Johnny but as you are in a wetter spot than him ( Liverpool) I would do as he says in regards to potting up to say a 5" pot and leave them outdoors to winter really sets in, that is regular frosts.
Then I would put them in your unheated greenhouse, I use a cold frame for such treatment as these as the name suggests do not get as warm as greenhouses.
What you do not want is the roots to get saturated then frozen.
I planted my seedlings out a couple of months ago so they should be well bedded in now and up to anything winter throws at us.
You do not say what sort of position you would have planted them had you planted them out but if it is a really well protected area and most definitely not a frost pocket you could risk your luck and plant out two or three. In this way you might get some flowers next year otherwise planting them out in spring will delay flowering if indeed you would get any.
The choice is yours!