It depends where you are based as to where works best for you, but you will see a huge variation in price between Docklands and other locations for obvious reasons.
In the Midlands / North Node4 have a few data centres now, and further South Gyron's new Centro facility looks pretty good.
Also it's not really related to what you asked but you might want to rethink your plan to run SATA drives in RAID 5 for VPS hosting - the performance might not be as good as you are expecting. I would consider RAID 10 for only 4 drives, as with RAID 5 / 6 you need a lot more spindles to get high performance. You might be OK with RAID 5 if you use faster SAS disks.
Finally be careful with your CPU specification - dual quad-cores could use a huge amount of power if you pick the wrong ones.
There is one that I was looking at a little while ago, COREIX, they are based in Stratford behind the High Street a few minutes walk from the NR and Underground station. I never visited them but they are listed as Stratford village which is an office complex opposite the shopping centre. If it helps to have access to your server and you are close by then for peace of mind I guess that will do but they are not great on prices. Poundhost certainly gives more bandwidth but they are based in Maidenhead and Milton Keynes.