Relationships with the people not the company are what counts as several people have already said. CEF are proving very good since I recently started using the local one, but I make the effort to meet them and have a bit of a chat. Wilts are good that way too, but their stock levels are a real pain at times. That might change now they've been bought though. Electricfix are useful for convenience and price on some stuff, and their deals are usually very good, including tools from the screwfix side of things.
I don't really use online suppliers for materials only tools. It's my experience that you can't beat being eyeball to eyeball with only a counter between you. That works well for good service and building a relationship. It works better than any other form of contact when things go wrong though. You can get fobbed off quite easily by phone or email, but that's rarely the case when you're standing right in front of them.
As for prices, well you can buy cheap materials, and you can buy decent materials cheaply. The saving must be weighed up taking into account the hassle factor of 'shopping around' which usually translates to using unpaid time to save a couple of quid.
I don't use the CU's from Screwfix, B&Q etc as I don't much care for the quality, reliability or ease of fitment. So I tend to stick with Hager and I can get a ten way board fully populated for £81:00 from CEF, £69:00 from Wilts and sometimes at a slightly lower 'offer' price from either (both inc VAT). To me it's just not worth the hassle involved in shopping around to save £25:00 or £30:00 on a board.
One other advantage I've found with using Hager is that when the customer says "But I can get a CU from B&Q for £50:00" I can reply, "Not one of this quality you can't as Hager only supply trade customers."