Think4 have an excellent range of sub £400 laptops, which are great value for money!
One example of these is the NEC i-Select M5410 WiFi Laptop, with 1.6GHz Pentium M 725 processor, 15" TFT display 1024 x 768 [XGA], 512Mb RAM: 60Gb Hard Drive, DVD ReWriter and Windows XP Home Edition. All this for £389.99 and £4.99 for delivery to add.
You can also earn Value Points and use them against future purchases at Think4.Com. Whilst you're there, sign up for their newsletter for a chance to win an Apple iPod!