I haven't looked into OEM pricing myself lately, but as an example:

Dell Inspiron 17 3000 1Tb w/Ubuntu: £419+tax Dell Inspiron 17 3000 1Tb+128SSD w/Windows 10 Pro: £619+tax

Isn't the SSD making up the bulk of the difference here?

Those ~£200 could get you a 1TB M.2 Samsung 970 Evo[0].

[0]: https://pricespy.co.uk/computers-accessories/computer-compon...

A 128Gb M.2 SSD is maybe £30?

Yeah, but PC manufacturers still charge like 3x the price at least for SSDs (or any component actually) when choosing/making your builds at their websites.

Windows Home, but not Pro. You get a bare-minimum media consumption platform with Win Home, no dev.

Windows 10 Home is fairly usable for many development workflows.

Ok, that makes sense.