I suspect a lot of it is due to UK customs regulations. In the US importing items under $800 is tax free once person per day (of course this does harm US businesses) [0]. In the UK the foreign seller has to register for, collect, and remit VAT (similar to sales tax) as well as deal with duties above 135 GBP [1].
[0] https://www.cbp.gov/newsroom/national-media-release/de-minim... [1] https://www.avalara.com/vatlive/en/vat-news/uk-post-brexit-v...