I bought a MESH earlier this year which has been fine. I think if you look at some of the PC mags, the likes of MESH and Evesham tend to do well with awards, etc., but I expect most makes/brands are much of a muchness. I think MESH is online only, but Evesham do have some stores if you don't want to buy online.
If you do buy from John Lewis then check out their 'price match' offer. Basically, if you can find the same PC (also TVs, HiFi, etc.) at a lower price anywhere else in the country, they will match the price, so you get the extended warranty for free. This is particularly good if you're buying a new TV as John Lewis sell these with a 5 year warranty. The only slight catch is that the cheaper price has to be from a physical store, rather than an online site.