Actually the Classic British seatbag was/still is Carradice.
Made since the 1930's they are an extremely durable and practical weatherproofed canvas/leather construction.
There is a '70's production British seat bag on my town bike and a Barley model like the attached pic on my Audax-ish road bike.

Not inexpensive to acquire but on a $ per decade of use an excellent value.
From a practical and useful design aspect, better than any "modern" design for my use(s) and decidedly Retro style.
They come in a variety of sizes to suit any need.

I acquired my Barley at a reasonable price new from a British cycle goods company a few seasons ago.
The bey also has listings of used Carradice bags, it takes a lot of abuse to render one worn out.

https://www.carradice.co.uk/

edit: Note the drawing of a British club rider on his way to an event w/ "sprint" wheels on carriers and kit in a proper seatbag "back when".

-Bandera