It always puts me off from buying something expensive because I wonder if somehow I could end up worse off (in terms of returns, or warranty or something) because I bought something that was X but only paid Y due to the voucher.

Realistically, I probably wouldn't buy a high-end computing product from Amazon anyway, unless is was notably cheaper than the specialists I'd normally buy from. Something like a £2000+ mini PC isn't the sort of thing the typical UK PC retailers I buy from would stock.

Like you can buy one at regular price, but 2nd and beyond get marked up 100%?

Preying on human psychology and the non-rational consumer. People are emotional impulsive creatures, and the marketing department knows this. If you think you're winning in some secret way only available to you (you're so special!), for a limited time only (so don't miss out!), it taps into something primal that turns off the rational part of our brains, and suddenly we're buying something we don't need at a price higher than we'd ideally pay.