Q.Asparagus fern – can I plant it in the garden? I live in the northwest.
Nothing else to say.

Certified GKH Gardening Expert
It is hardy in US planting zones 9-11 and you are listed as Cheshire, England, which, according to Internet data, doesn't typically fall below freezing in the winter. So, yes, I would think you could plant it outside as a groundcover. If not, you could plant it in a container outside for the summer and bring it inside during the winter as a houseplant.
Here is more information:
https://wimastergardener.org/article/asparagus-fern-asparagus-densiflorus/
https://www.gardeningknowhow.com/ornamental/foliage/asparagus-fern/asparagus-fern-care.htm