For emulsion, Wilko budget brand is very watery and you need lots of coats. Wickes own brand is often available in 15litre tubs, which can be good value if you have several rooms or lots of ceilings. Wickes often have a Reduced display near the door.
For gloss, Dulux Trade is very good. White paint is always cheaper than colours. It should have blue lids which indicate the anti-yellowing formula.
It does rather seem like cheap paint is cheap for a reason.
For indoors gloss paint, you can use the water-based acrylic primer undercoat, which dries fast and is quite soft. For outdoors, I'm sure oil (white sprit) undercoat and aluminium primer are more durable.
Keep searching online, Wickes, Homebase, B&Q, Wilko, looking for special offers, especially around sunny weekends and bank holidays.