I deal with Amazon a fair bit here in the US. Sure they and/or their sub vendors make mistakes now and then, but they are more than acceptable at correcting them to keep you a happy customer.
Case in point, I just ordered a $144 rug from one of their sub vendors. The SV contacted me just before it was supposed to be shipped and said it was the last one they had in stock and too dirty to ship.
A day later I got an email saying it had shipped. The problem is Amazon's order tracking system is over simplified and too automated, and doesn't even let vendors alert them of such things.
I went through a bit of grief and confusion over it, worrying if the dirty rug had been shipped anyway. The vendor ended up apologizing and taking blame for not fully clarifying what was going on. They just offered me a 30% discount while I wait for them to get more stock, which they say will delay the order about 2 and a half weeks max.
My point is such problems are usually resolved adequately if you communicate with them. I've yet to see a situation where their sub vendors or Amazon won't offer you an exceptional deal or a refund for any trouble the customer is put through. If you can't contact the sub vendor other than to ask for refund or reorder, call Amazon and explain the problem. They will email it to the sub vendor, whom will email you right back, it's a pretty quick process.
That said, I've read good things about Scan in the UK, and I know they have great service and reasonable prices. I'm also not sure if Amazon UK has the same quality control over the order processing and customer service in general as Amazon US. I know Jeff Bezos is a stickler for details though, and a big part of what made his Amazon empire grow so big is his attention to customer service.