As far as I know Axminster don't sell pen jaws (although they do sell a variety of jaws that would do the job).
The only dedicated pen jaws I know of are made by Robert Sorby to fit the Patriot chuck (they also fit the Nova chuck)
There are many suppliers of both Patriot and Nova chucks. Stiles and Bates, The Toolpost, Turner's Retreat, Peter Child, Yandles and Snainton to name but a few.