How about an Inspiron 620 MT? The third one there is £429 (VAT and shipping included), and these are some of the things it has:
Genuine Windows® 7 Home Premium, 64bit, English
Intel® Core i3-2120 Processor (3.30GHz, 3MB)
4096MB Dual Channel DDR3 [2x2048] Memory
DVD +/- RW Drive (read/write CD & DVD) with DVD Burn software
500GB (7200rpm) SATA Hard Drive
Graphics : Intel® HD
Integrated 5.1 High Definition Audio
Integrated 8-in-1 Media Card Reader & No Floppy
Dell Wireless 1502 (802.11n) WLAN Full-height Mini-Card
Dell IN2030 20" (51 cm) HD WLED Widescreen Monitor VGA, DVI-D (1600x900) Black UK
Dell MS111 USB Optical Mouse (Indigo)
Dell USB Entry Keyboard - UK/Irish (QWERTY)
Doesn't come with speakers, you could just pick some cheap ones up. It sounds like a decent deal to me, though I'm no expert on systems, so someone else should tell you how good it really is. But it should be able to hold you over if you aren't doing anything intensive.