I have used Dulux mix to order paints for about 15 years and knew that my local B&Q was converting to Velspar.
I have recently painted the boys den in a dark grey and had Velspar premium paint. It went on really easily and the coverage was good and even. I did two coats and it was covering up a light blue dulux colour. I would definitely use it again.
I also used to use Leyland acrylic primer and top coat for skirting and architrave which I used to buy from Screwfix. It is water based gloss and has stayed a spectacular bright white for 2 years. You have to thin the top coat with a bit of water in a paint kettle, and you do need several coats but it dries in couple of hours so you can re-coat several times in one day.
Last year I couldn't get hold of any top coat from Screwfix nor could I order it from anywhere. I bought Valspar Premium Gloss base which they would normally add colour to but I wanted brilliant white. It is a primer and top coat in one, you do have to add a little water to it, again in a paint kettle and start somewhere that will be behind furniture so you can get a feel for it. It is a fantastic product, and I would highly recommend it.
I have used in the past, B&Q's own paint, Wilkinsons, Crown, Dulux, and I would recommend Valspar. If you can't get a good finish you are either using the wrong roller or brush or you aren't putting enough coats on 