While the tops of container plants may be quite hardy the roots are vulnerable to frost. Here are some easy steps you can take to protect them in cold weather.
Bubble-wrap, String, Newspaper, Plastic-bag
Tie a double layer of bubble-wrap polythene around the pot to give a layer of insulation against several degrees of frost.
For more protection stand the container in a plastic bag...
...and fill with crumpled newspaper to create a thick insulating layer...
...tie the top loosely with string or a platic tie. Keep the plastic off the leaves as it can freeze to them causing cell damage. Never cover the plant with plastic.
Growing a Lily in a Container