Have you checked out the Ubunbtu Certified hardware site?
Ubuntu Certified hardware
For desktop, laptop, and netbook computers:
I'm currently looking at a (basic) laptop to use with Linux, too.
- The one I'd quite like to get isn't available in the UK -- the System76 Lemur Ultra-thin. But they do ship form the US to the UK, Australia, ... Of course, then there's shipping and duty :-(
- There is a Linux-based (small) laptop/netbook available in the UK (only, I think), called the MeeNee. It's not very powerful, but it is pretty cheap. Prob. not worth the efforts of shipping elsewhere.
- Currently looking into the assorted Asus models which hit no issues with Ubuntu during testing, even if it means installing Ubuntu myself.
PS Dell is charging £1,079 for the XPS 13 here in the UK, approx $1660 (US and Australian) at today's exchange rate. cf. $675 (approx £440) for the Lemur. While I haven't compared their specs yet, I do see they both use Intel i5 (or i7, as an upgrade) processors.
PPS In the US, it looks like the XPS 13 is only $749.99 (down from $999.99), so we've got a poor deal over here...
ETC More about the Lemur
Hands On With The System76 ‘Lemur Ultra’ Ubuntu Laptop