Cut them back a little once they stop blooming, but leave long stubs to collect snow or hold mulch for protection over the winter. Cut the stubs off in spring as the new growth comes up. You can keep them a little shorter and more compact by pinching out the growing tips every 3-5 leaves over the summer, but stop that in late August so they can form flower buds.

Dont hold your breath if they were very small plants bought at a GC this summer? My 3 didnt survive last winter. Have you a green house if so you could over winter them?

The earlier and smaller the flowers are, the greater the likelihood that they are a variety intended for landscaping--as opposed to florist's use--and so quite hardy outdoors. After the monster winter the UK had last winter, I wouldn't be surprised at florist's mums succumbing!