I grew this tree fern several times, and each time I killed it by taking it out of its pot and transplanting it into what I thought were good conditions (moist, shaded, rich soil).
It makes almost no root system, so is best kept in a pot and potted on as necessary. It grows quite slowly. A few degrees of frost and it seems to be enough to put paid to it.
As you are in Berkshire, which is not the warmest of counties, I'd keep it in its pot and bring it in as soon as more than a degree of frost is forecast. The greenery does die back in winter and then it sends up new fronds in the spring. The 'trunk' is simply the base of the old fronds.
I had a friend in the UK who grew a magnificent specimen in a 15 inch clay pot, but it was in a courtyard garden and protected from frost.
The big tree ferns you see in gardens are mostly in the far south-west, and even in these positions I think they protect the crowns from time to time.

The fronds will die down if frosted Sally or if kept above freezing they should survive with the fronds intact, I have 4 bedded out in derbyshire and find they come through our winters ok with a little protection, I put a handful of leaves down in to the crown and during the worst months wrap hessian around the trunk, I find they do make a really good root system and would have trouble trying to put a trowel in near the base of the Tree fern, they are very slow growing though and the best way to see one with trunk is to buy it that way, they grow a few inches a year at the best

Thanks for your question Sally...all the answers have helped me with my very small one...I shall keep it in a pot well out the way of freezing temps during Winter then in the summer place it in the shady side of the garden.