I think I was very lucky in that my uncle was 'in' the trade.
anyone can walk in to the depot and buy stuff but my impression was that the prices are not set at all and vary wildly on who you are.
I had to plan it myself, they didn't send anyone round, but if I was you, I'd get B&Q or MFI round and get them to plan it for you, give you a nice picture, then you can do what you want, as long as you know you want say, a 600 unit here a 400 wall unit here etc... you can do the rest and make a list.
Someone in the trade who's 'in' with them can then order it for you.
My uncle got mine from the Avonmouth depot in Bristol and they were more than helpful in delivering it, bringing new parts that had broken in transit (this was one wall unit that had crushed slightly).
To show how the prices vary, the end panels when bought through my uncle in the main transaction cost £29 each. When I'd nearly completed the kitchen some 4 weeks later (I did it on weekends and nights) I cut the end panel too short and needed another one, I just went in from work and asked for one, he asked if I has an account, I said no and he charged me £71!!