With that kind of budget you can get a very nice machine.
Which? magazine January edition recommends
the Acer Aspire 7735Z from ebuyer. As a PC freak I am usually bemused by Which? articles on the subject but appreciate they tend to write for the average man in the street - ebuyer typically is very highly rated as an e-tailer. Another option I usually mention is Dell: not that their machines are the greatest but you do get as much support as you are prepared to pay for.
Going back to that Acer - a spec of 4Gb RAM is spot on. 2Gb may suffice if you are really penny pinching and 4 is more than sufficient for most stuff in 2009 and 2010 and likely 2011. Also a 320Gb hard drive is pretty much the sweet spot - 250Gb is penny pinching and probably OK, unlikely you will fill 500Gb in 3 years unless you start storing loads of video (typically 1Gb/hour). On CPUs - you pretty much get what you pay for. Everything at that kind of price point should be dual core, 64 bit - dont settle for anything less. I have a personal prejudice for AMD solely because I see Intel as the big ogre who will run riot unless there is some competitor and AMD is the only viable competitor at the moment, so I would look at stuff like the
Acer Aspire 7535G. For Operating System you almost definitely want Windows 7 - (nearly) all laptops will come with some flavour of Windoze and I cant see any logic in going for anything else. Then again my poison is Linux so others here may be more specific.
As you start comparing deals you will go nuts with model numbers - as you can already see in the para above the 7735Z has subtly different specs to the 7735G. You will run into the same issue with any manufacturer where apparently the same machine has different specs at different shops! Take your time - if it looks like an exceptionally good deal you are probably missing something!
(In the highly unlikely event you are willing to go for Linux then
novatech are the only guys I know who sell laptops without a 'free' copy of Windoze, which results in about a £70 saving! Let alone saving another £50/80 on 'essential' security suites for virus/firewall/spyware etc!)
For sure I would advise against the current 'darlings', Netbooks, as the only PC in the house. They have undersized keyboards, small screens, underpowered CPUs, small disk capacity and small memory size so despite the fact they are 'cute' I seriously doubt their suitability for general purpose/household computing that will still be able to do what you want in 2011.