Just gone through all this myself. In the end its all fairly subjective but Dell are currently offering £100 off some laptops bought off their site (although this may end tonight 31 May) and a web search (look for dmxdimension.com) will find a site offering codes for a 5% discount off HP laptops, bought off HP's own site.
For what it's worth, I went for an HP machine; ordered today, they are threatening to deliver it tomorrow!