I have tried the supermarket own brands Tink, and find them not as refined as Brand named ones, I use Homepride Self Raising for my cake baking, it always works really well!.....worth paying the extra!....
I think if you put it through a sieve so that the air gets into it and it becomes nice and fine there is really no difference except of course for the price. My cakes and puddings always turn out all right whatever brand I use.
The main difference, as welshlibra says, seems to be that the 'brand names' appear to be more sifted,light & powdery. So for sponge cakes it may be worth buying Homepride, for heavier (eg fruit) cakes I can't see it matters.