The only complications about dealing with a builder's providers are that they normally deal with commercial builders, hardly mark the price on anything, are DEAD slow and have potentially embarrassing security measures. On the plus side, advice got at such a place is normally of higher quality than that got at tool rental places.
Many people expend great energy trying to convince the staff at a builder's suppliers that they are "trade" and deserving of the trade discount. I watched in pain one day as a small lady tried valiantly to convince the staff that her builder was buying the material and had simply asked her to pick it up. I really don't think this is worth the bother. You can usually acquire most if not all of the trade discount by bargaining, paying cash or by unsolicited deductions by the sales staff. Note that this is what happens in a "real" builders providers, I assume that in large DIY warehouse places "the price is the price".
If you really are after the "trade" price, the trick seems to be to turn up in working clothes, mention nothing about trade prices, pick up the stuff you want and pay cash. Buying LOADS of stuff seems to convince them as well. Your plumber or electrician etc. may also suggest that you "tell them I sent you".
The lack of prices on anything is a far bigger problem. This makes it very hard to shop around. It is usually impossible to find an assistant who can both help you and knows the price. With a bit of luck your item will be marked, otherwise just allow plenty of time. Remember to check that VAT is included.
The most time consuming part of the trip will be the "soviet" style sales procedure. You queue for ages to hand the goods into one of the very rare assistants, they tap everything into the computer / till and hand you a two part docket. You next queue at the cash desk, pay for your purchases and receive a brightly coloured receipt. This receipt is very important, it will enable you to get out of the car park.
Most of these places are basically warehouses with large yards. As such, security is a problem. They expect everyone leaving the place to stop at the gate and hand over the receipt. They will then quickly check that what you have is what's on the receipt. In order to avoid potential embarrassment, I recommend keeping receipts if you intend going from one such place to another.
We caused consternation one day when we drove past the security post without stopping because we had not bought anything. Making cheerful "empty hands" gestures seems to work in this scenario.
Well that's it. Hope our experiences were some help. Add an entry to the guestbook if you have any comments.