Look at the following scenario. A new player walks into a GW store, and proceeds to spend £300 on a starter box, army book/codex, and miniatures. They get to the till, where the staff member says, "You'll need some superglue to assemble your models." "Can I buy some here?" asks the customer. "No," says the staff member, "You'll need to travel down to the nearest DIY shop that's around 3 miles away to buy some."
"...But Poundland is across the road."
And thus GW is accused of being in cahoots with Big DIY...

It's correct to say GW supply it because if they didn't they'd look stupid, but at this point the only one looking stupid is the consumer. I'm sure if we look at every GW in the UK on Google maps we can find, within ten minutes walk, a cheaper location to buy 80%+ of the tools GW sells at a premium price, and the other consumables are also a lot cheaper elsewhere. Quite often the same tools in fact, or superior. £4 for 5g of superglue in GW? Same amount in Poundland gets you 80g of glue. There's things like paints and epoxy that won't be so accessible of course but anyone savvy enough can just go online for them and find them cheaper with ease.
End of the day, GW's tools are a con. Anyone with the information knows the alternatives are cheaper and as effective, or better. If GW think for one minute they'd realise that the overpricing can't last forever - the bubble will burst.
I honestly think GW is doomed if they keep up their isolationist policies. The model they have barely worked twenty years ago - in the information age, pretty much all they can sell viably is miniatures in their setting. Tools? Cheaper. Rules? Easily pirated, slow to update, not available in electronic format like practically everything else (lo Kindle) and competing companies have elected to throw rules up for free. Oh, and none of the rules are given beta testing in public. Fluff? Well, when fanfiction.net has better writers than some of yours (Ward, Goto), you're in serious trouble. When something called Harry Potter and the Thousand Sons has more respect for the background and the playerbase than your canonical Grey Knights fluff then you're in serious trouble.
Games Workshop claim to sell models first though. If that's the case, why do they insist on providing the secondary materials as second-rate, and the primary materials of increasingly questionable value for money and overall quality?