I was looking at buying a Beelink NAS and paying for a Windows license when I'm just going to install proxmox on it (I think) definitely counts against it.

I wouldn't worry about it. The cost OEMs pay for those Windows licenses is negligible for a "low-end" device like a simple NAS box.

How much do you think Beelink pay?

I'm expecting say ~£20, which is significant.

The price for Win 11 home on https://www.microsoft.com/en-gb/d/windows-11-home/dg7gmgf0kr... is £120.