How about something like this. It has a good CPU, graphics card, 2gb Ram and it is SFF. If you can afford to go about £20 over and will be gaming then I'd suggest the X800 GTO16.
CA-021-AN Antec Aria Micro ATX Cube Case - 300W PSU (CA-021-AN)
£59.30 £59.30
MB-119-AS Asus A8N-VM CSM Micro ATX (Socket 939) PCI Express Motherboard (MB-119-AS)
£54.95 £54.95
MY-006-OK OcUK Value 2GB (2x1GB) PC3200 184pin DDR Memory Dual Channel Kit (MY-006-OK)
£89.95 £89.95
CP-121-AM AMD Athlon 64 3700+ San Diego 90nm (Socket 939) - Retail (ADA3700BNBOX) (CP-121-AM)
£137.95 £137.95
CD-028-NE NEC ND4551 16x16 DVD±RW Dual Layer ReWriter (Silver) - OEM (CD-028-NE)
£27.95 £27.95
HD-048-WD Western Digital Caviar Special Edition 200GB 2000JS SATA-II 8MB Cache - OEM (HD-048-WD)
£54.95 £54.95
GX-054-PC PowerColor ATI Radeon X800 GTO 256MB GDDR3 TV-Out/DVI (PCI-Express) - Retail (GX-054-PC)
£89.95 £89.95
Subtotal £515.00
VAT £90.13
Total £605.13