I imagine very few employees of these retail giants actually know what half the stuff they're selling does.
Just because a PC costs £700, does not make it worth the money, especially when you buy it from the purple place (I fell into this trade a few years ago, £700 "gaming" pc, with a £40 graphics card)
A more expensive HDMI cable will be better, but the difference is hardly noticable between a cheap one and an expensive one unless you're after a really long cable, it still does not justify the cost.
I've got to the point now that, with the exception of external hard drives, I'll go into a shop, look at what product I'm interested in, then most of the time I'll buy it online, you don't get the uninformed salesmen trying to sell you the most expensive thing, you can read 100's of reviews detailing the positive and negative about it and 9/10 times its cheaper because the company isn't paying the uninformed salesmen to up-sell the more expensive version as "better"