Apple is really the only UK seller that sells refurb Mac Pros on a regular basis I would say that the two sites you mentioned sell them relatively infrequently, but are worth checking (as you already are!). Theres also Cancom UK, as that has a refurbished section also, theres a few sites that specialise in second-hand machines, some of which say they are refurbished machines just try a Web search for refurbished mac and you should be able to find a few.
I would also say all premium or specialist resellers like Jigsaw are worth checking (
http://www.jigsaw24.com/refurb.aspx) as many do offer refurb machines but the supply of these isnt what I would call regular, especially for Mac Pros. Even if theres not a section on their website, they would be worth contacting.
However, whenever new models come out just about all Apple resellers are worth checking out as this is when theyre trying to shift old stock and ex-demo models (until new models are released its not much sense for resellers to flog off demo units) and the best idea to hit the phonelines.
Obviously, we dont know about know when the new machines are coming out, but when they do, but that really is the best time to try and get what youre looking for!