The parts are functional so I think one has to judge that in relation to the price of the bike, and of the price of spare parts. You can take the headset as an example, it's about as low quality as they come and this type of headset would only normally be found on the cheapest of bikes, but there you have it on an £800+ bike. It works OK but it's unnecessarily heavy and functionally inferior to cartridge headsets. I replaced it with a cartridge headset that weighs 95g (the standard steel headset is 155g and the alloy version used on the ti models 120g) the action is now much smoother and I won't have to replace worn cups.
Another example: the bottom bracket retails for around £21 but the same BB is sold without the Brompton branding in Germany for around 14 Euros. Similar story with the chainset, it's a slightly altered bottom-of-the-range Stronglight model but sold for a much higher price.
So the parts are functional but it's unheard of for such cheap parts to be used on a bike of this price, and replacement parts are often heavily marked up. Strangely, and conversely, some of the spare parts are very cheap, e.g. the chain tensioner. One would have to feel that the overall quality of the product justifies the tendency towards inflated prices.