I am sure the recommendations here are good, particularly if you want a specialist or custom built machine - and I will certainly have a look at Scan. But for a standard product, I prefer to buy from a company with a physical shop that I can walk into and speak with actual human beings - and the manager if need be. I am not interested in spending hours in a telephone queue to speak to an adviser or to have to engage in a lengthy email discourse.
I want good customer service from intelligent and engaged staff, and easy resolution if there are any problems. So John Lewis or Apple for me. Currys have poor after-sales service in my experience, don't know about PC World.
John Lewis products will price match anything that is on genuine retail, plus you get good service, both before and after sale.
(I don't work for John Lewis or Apple).