We Brits seem to get ripped off on just about everything: compare the cost of electronic and electrical goods with those in the US too. Or US companies such as Lands End, who just double the $ price and then put a £ in front of the number! I've just been to the US on holiday, and have stocked up on FQs for my current quilt, all costing about half what they would have here.
I find I can buy 3 or 4 yards of fabric by mail order without attracting the attention of Customs, but more than that and delivery is likely to be very delayed and include high charges for tax and fees. US mail rates have gone up quite a bit in recent years, and there's no longer the cheaper surface mail (it exists here, but not in the US!) so it's not always worth ordering from there any more - depends on the exchange rate. I have a friend who comes over to the UK once a year, and she often brings me some fabric too.
There's also the issue of supporting UK quilt shops: another reason I buy much of my fabric here. I don't think the high prices are down to them, and I value their presence.