I have found that the paper size 400 millimetres by 300 millimetres is used by some printmakers in England.

It appears to be regarded as a traditional size for paper for drawings and for some frames. Rather than A3, which is a similar size, yet not so long a time a specified size.

Yet it appears to be a French size.

The name Cloche (Bell in English) being because at one time (some of?) the paper of that size had a watermark of a bell.

https://papersdb.com/french/cloche

Yet the metre as a unit of measurement is not that old in terms of art traditions.

I am wondering why this size is used in England.

William