I'd personally avoid Aluminium Warehouse.

I ordered two sizes of bar in my last order, and they substituted both sizes. The one size they asked about substituting, I explicitly told them was unacceptable, but next thing I knew was I'd apparently "accepted the offer", and the order was dispatched. When it turned up, both sizes were wrong and despite emailing and phoning, they've yet to do anything about fixing it.
So I'm currently sat with over 30metres of bar that is no use to me.