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.