Although 10a 215 was produced in England, the scribes made use of imported paper for Parts II and III. How do we know? From the watermark that is visible on the pages. Below is what the watermark would have looked like before the sheet of paper was folded.
What were watermarks? Generally, they are the identifying marks of paper makers. European paper was commonly made from rags during the medieval and early modern eras. First, the rags were beaten and the resulting pulp soaked in water. The paper maker would then dip a wooden frame with a sort of wire mesh bottom into the pulp, and gently shake it to even it out. The pulp would then be pressed to squeeze out the remaining moisture and left to dry. In order to identify their products, paper makers would often twist wire around the mesh screen, which resulted in thinner paper around the wire in the shape created.
The most useful resource in identifying watermarks is C. Briquet’s Les filigranes, published in 1907 and available at Hathi Trust here. (It is in French, so make sure you know what the French word is for the shape you’re trying to identify.) Briquet catalogued over 16,000 European watermarks found in paper that was made before the year 1600. However, Les filigranes does not (nor could it) contain all watermarks created before 1600, and there have been later publications that attempt to fill those gaps.
Peter Kidd has identified this particular watermark, found in Part III of 10a 215, as similar to Briquet no. 14232. Briquet no. 14232 links to a paper maker in Brussels around the year 1440. If this is correct, we can date 10a 215 to no earlier than 1440.
For more on paper making and watermarks, “Making Watermarks Meaningful: Significant Details in Recording and Identifying Watermarks” from the The American Institute for Conservation is a good resource.