The Sinden guy sounds British so he's going to have much higher production costs probably, the wild difference in pricing vs. that Chinese company (for what you get) will be down to production at scale costs as I understand it, my source is a few years old video* so maybe its different now but if you're in China you can get stuff made in Shenzhen for buttons even in tiny quantities, whereas over here you're paying way more even for larger quantities.
Even after all that though... it does seem like the Sinden guy is just charging more because he can. I sell TVs, the brand I work for has an entry level TV that overall gives you a better picture than one that Samsung charge nearly double for, and they have it as part of their mid range sets, so the Samsung should be quite a bit better on paper. Sometimes its not much to do with the BOM or the overall production costs and more down to R&D, marketing and logistical costs that dictates the price required to get the return on investment they need/want.
*It was on Strange Parts channel, but I looked and none of them ring a bell, in the video he talks about how you could buy parts in a quantity of 1 locally in Shenzhen and it was cheaper than when you ordered in qunatities of 10 or 100 remotely.