This the stuff I use all the time (unless customer specifies a different brand)
http://www.wickes.co.uk/Trade-Emulsion-Colours/Trade-Professional-Matt-Emulsion/...There are various colours and they do silk as well.
The do a cheaper Trade version that is also pretty good but I pay the few extra pence per litre for the pro type.
I have not compared the various Trade paints side by side. So I can't say if/how much better the pro trade is vs standard trade?
I think with a lot of painters there is a touch of brand loyalty, I know some decorators that will not even try a paint if it does not have Dulux written on the tin. I think this is the reason some brands charge as much as they do, they know that there is some mug who is willing to pay over the odds for it.
There are also some rubbish paints out there, so it is a case of trying a few different brands and seeing which one gives you the results that you are happy with...