Totally depends on what you want to do on it.
If you want to use the internet, email and use Office (Word etc) then just buy yourself a Netbook - they're small and cheap, you can get a decent one for under £300.
But if you want to do any gaming or using more powerful applications like Photoshop or video editing, then you're going to need a laptop with a dedicated graphics card and more memory/faster processor and then even your £400 budget isn't going to get you something good.
Apple's are expensive, but so are PCs if you buy a decent one. :lol: There's no way you could build a PC that matches the spec of an iMac for anything close to what they cost.