Dell are well recommended, and have a good presence in the commercial market, which should speak for the quality of the product.With Dell you have to look out for their special offers, usually in various media publications,good savings are usually offered with free delivery.
Mesh and Evesham have a presence within the PCA forum, so any problems can soon be remedied via this channel. Again, both companies and products have a good recommendation ratio.
For the 'see before you buy' approach, PC World carry a fair range, with help on hand should any problems arise.Staples and Office World carry a very small range of branded computers,and these are usually available on special offers.
So the market is really open, and the above suggestions should result in a price range of £399.00 upwards.