You don't need to keep the camellias under cover in the winter they are tough as old boots, the fact that the leaves are curling up possibly means its died of thirst and watering now is little to late, what you need to do is take it out the pot refresh with some decent ericacious compost and see if it responds, do a bark scrape to see if it's green under the bark, if it's a dull browny yellow colour then it's on it's way out, for the future keep outside in a decent pot feed regularly through the growing season and water well through long dry periods in the summer whilst the flower buds are forming, I have seen camellias in pots for twenty/thirty years or more.

There are one or two slightly tender varieties, but most readily available ones in this country are, as Julien says, bone hardy. The connection with frost is this - if you plant or stand your Camellia where early morning sun can hit it in winter, before the frost has had time to dissipate off the buds, then the buds may drop off. If its not placed in early morning sun, this isn't a issue, so frost or cold on its own is no problem for the plant.

Thanks guys great info. I thought as much, that I started to water too late... Will follow advice this week and hope I can save it.