Hi,
I contacted some UK custom PC shops and only one got back to me - nsys.co.uk - and I really recommend their customer service! :-)
The machine they have put together totals £1018.27
The motherboard is a "Tyan Thunder K8SE (S2892G3NR)" with a single Opteron 250 2.4GHz Processor. Since this is a server-grade motherboard, it requires an expensive (£200) PSU - a "24Pin 8 Pin Enermax Server Grade".
The "128MB Asus Radeon X300SE Extreme" appears to use the r300 chipset which is being steadily more supported by X.org
I am considering "Seagate 250Gb NCQ" harddisk, and the NCQ features are not yet supported - http://en.wikipedia.org/wiki/Native_Command_Queuing#Operating_system_support - but should be soon.
I wonder about firmware status of CD/DVD Optical disk drives and sound cards.
Does this list have any recommendations and comments on the above? :-)
When I get this machine, I'll write up a report about what I did and why as per http://www.fsf.org/campaigns/supportlinuxbios.html