how do you protect the tree fern in winter from frosts?

Hopefully your plant will be one of the higher altitude forms of the species (most are now thankfully) and can take a surprising amount of frost though not over long extended periods. I did see a certain Mr Titchmarsh encircle the trunk of one he had at his garden at Barleywood with chicken wire and stuff it with straw. The fronds could be left on with just a handfull or two of straw in the top to protect the growing point.

Fractal is right, I do this with mine and haven't had any problems. Someone on the site suggested tying up the fronds to cover the crown as well.

Mine is still quite small so has a nice coat of green frost resiliant fabric, and some dead leaves in the crown..and some turves round the trunk for extra protection....

My daughter in who lives near Bath wrapped her Tree Fern up in fleece the first year.Forgot last year and thought she had lost it but it did survive. She will wrap it up again this year.

As long as you keep the crown dry and frost free you should be fine, this is where the new fronds come from.

Bubble wrap is also used but understand the straw method is preferable.
Good luck and let us know how you get on.

I agree, don't use bubble wrap. If condensation gathers on the inside it will freeze and potentially damage your fern