Age old question...trade or retail.....
Trade paints tend to cover better than retail...this is because painters need to be able to rely on a paint covering for cost purposes..DIYers can afford to spens a few more hours painting a room for the third or fourth time....when your doing it for a living....you cant.
Retail paints tend to smell a little nicer, and have smoother flow qualites to assist the novice decorator.
In order for a paint to have these qualities paint companies have to leave somthing out....and its normally the pigment...hence less opacity.
As people have mentioned above, the word 'trade is deceiving.....there are quite a few outlets selling 'trade' paint....normally unbranded in big buckets....for about 4 quid....forget it.
Retail paints have a false body...open a tin...turn it upside down and the paint will stay there....give it a stir...and it turns to liquid.
Dulux and Crown are two of the best paint manufacturers in the uk...certainly beating people like Jonstones and Leyland....that said..I would rather use Jonstones or Leyland paint than Dulux or Crowns retai paint..which is a bit on the thin side.
I have always said to people..if it is going to need two coats for it to cover anyway....then just buy the cheapest....within reason...id certainly avoid home brand paints and 'supakuvva' types from the local pound shop!
Another thing to be wary off....'Brilliant whites'....many of the unbranded and budget brands call their paint 'brilliant white'...in most cases it isnt.....its a dirty white....this is because here is no British standard for brilliant white...only 'white' which is 00-e-55
http://www.britpaints.co.uk/Site/there'stradesupe.html
Have a blimp at that.
Hope that helps