A quick look on Ebay for 'daily deals' (linked from front page) will find some daily, or weekly, deals costing from around 225 up.
Looking at laptops that are above minimum spec so won't need replacing for a while, there were a couple I saw with 4GB of RAM and generous hard drives, one (Acer) at 299 and an HP at a little over 400 pounds.
Unless there are really strong reasons for choosing some others (because of software you want, or for playing video on a big TV needing some special connection) I would have thought a 400 pound laptop would cope with majority of needs...
My main reason for mentioning the HP was because some here on MN have reported less than ideal experiences with Acer, so you might want to spend the extra for a 'big name'. However other names to consider should include Lenovo (IBM), Compaq (HP) and probably Toshiba (been around the longest, but maybe they have lost the edge). I know some people swear by Sony but I think they may be a bit overpriced (similarly for Sony radios/music systems/TV, and the fact they have Sony stores to keep prices high, add to my belief!)
Maybe worth checking John Lewis (.com) as you may find a longer warranty and possibly a price match.
Also, worth visiting JL, Comet, Currys branches to check the keyboards before parting with your cash. Some people find they love or hate specific keyboards. I would tend towards 17" laptops with separate numeric pad.
I rarely actually use the laptop keyboard (4 laptops here at present) because each is different, some don't have a numeric pad, function keys and home/ end/ page up/page down etc are often awkward... so at home I plug in a full size USB keyboard, and an optical mouse. Cheap option (keyboards from 4 pounds, multimedia a little more expensive, but give volume control etc without touching the laptop) and standard layout, prevent spills from getting into the laptop, and I can place the laptop further with the screen closer to eye level, but those are personal choices, of course.