Don't know about an Amazon sale - I think they would say they're cheap all the time. My personal preference would be http://www.novatech.co.uk from Portsmouth. You can build on to a barebones system from them and turn it into whatever you want. Delivery is quick and prices are very competitive.
Prices usually start dropping after Christmas, many retailers make a large part of their annual profit during the run-up to Christmas; this year may be somewhat differant due (in part) to the Thailand Floods, the prices for hard drive are going up by the day.
all good UK component stores, often having short offers, i've never really seen anything around christmas especially though. Amazon themselves may not have the volume, the independents that use amazon as a front won't have the volume and have to pay amazon as well. The above stores are often cheaper.
Actually I've just checked and am surprised at amazons prices, not to bad really, shop around though. Lots of bundle offers on scan for instance.