Acer are very hit and miss it seems. For most people they work just fine, others have constant problems. But then they are cheaper than most other manufacturers, up to you whether to take the risk or not.
I have to say though that I haven't looked much at the bigger computers, and not in the £1300 range (Mine was ~£800 and that was as far as I wanted to go), so can't help you much there.
The Lenovos are, from what I've heard, very well built. I know nothing of the bigger XPS models, just the M1330 which isn't what you're after.
You should go for 3 or 4gb ram (32 bit operating systems can only handle 3 IIRC, so don't go for 4 if you can't use it all), 2.2ghz + dual core processor. Again, I'm not very clued up on graphics either (Tbf I'm no guru in other aspects either), but
this site could give you a decent idea of what's good or not.
The Dell Inspirons aren't very flashy, but would be decent value for money. Brands like Sony, Alienware etc. are very high quality and really cool, but you pay a lot for that extra quality. HP/Compaq are priced rather well too, but my personal experience of them isn't the best (Doesn't say much though, just one laptop bought 3½ years ago). Other than that I can't help you much with brands.
I'm sure some more useful people will come in here soon