A lot depends on what you would use it for. What to consider:
Size & weight - look for something ultra portable if you're going to be carrying it around. If you're just going to use it in the house then go for the max size possible.
Hard Drive - 500GB is enough for most people unless you're going to store tons of music & films.
RAM - depends what version of Windows you are running. Personally I would go for 64 bit so you would need at least 4GB of memory.
Processor - go for a newer processor. For that price the best you are likely to get is an Intel i3 processor, maybe an i5. Check the benchmarks here -
http://www.cpubenchmark.net/cpu_list.php.
Graphics Card - don't bother unless you're into online gaming as they push the price up drastically. Integrated Intel graphics is ok for most people.
Check the battery life, you need an integrated card reader (most have them), minimum of 3 usb ports, must have a DVD rewriter (some of the smaller ones don't), do you need bluetooth (it's handy to connect phone to laptop), consider one with a HDMI connection, do you need an integrated webcam & microphone.
As far as make is concerned you could ask 10 people and each could give you a different answer. There are some to avoid - Advent, Emachines, Packard Bell, Fujitsu. For some reason Dell & Acer laptops have had a lot of problems recently. Makes that have a good reputation - HP, Asus, Lenovo, Toshiba, Sony.
If you go to places like Comet or PCWorld you're not likely to get a good deal. I've always bought off ebay and never had a problem - here's one example
http://www.ebay.co.uk/itm/HP-Pavilion-G7-17-3-HD-Screen-6GB-RAM-750GB-HDD-Bluetooth-HDMI-/271041028795?forcev4exp=true#ht_3939wt_1344. Also have a look at the Argos Outlet on ebay as they sometimes have bargains -
http://stores.ebay.co.uk/Argos-Outlet?_rdc=1. One of the links above was for Sainsburys, they do have some bargains on laptops instore - all about being there at the right time.
Hope this helps.