DO NOT buy cheap paints; they will be made from lots of fillers and thickeners and very little titanium dioxide and other quality additives. There’s a reason why they are cheap and TiO2 is expensive nowadays.
Go to a Dulux Decorator Centre, get a loyalty card and enjoy the 10% discount. You will also get free, experienced and friendly advice and quality equipment. On the face of it Dulux trade paints look more expensive than the garbage you buy at B&Q and even the Dulux “amateur” paint at B&Q but it’s made for professionals who want to put on two coats not four and who appreciate a paint that can be pulled out along an edge. With the discount this probably works out cheaper in the long run than using cheap paint. It’s certainly less effort and time wasted.
But some people will always operate on “optical pricing” no matter how many times you tell them. Sigh.