We're having a fairly major redecorating push ATM. I have always bought paint from our local old fashioned iron monger, who stocks Dulux, and mixes colours on demand. Wifey bought a 5L tin and said to me how expensive it is, at nearly 39 pounds a tin, or 22 pounds for 2.5L. Just checked the B&Q site, where 5L is 38.99 and 2.5L 24 pounds.
No point to message other than support your local shop wherever possible. Yes, we could probably buy the paint cheaper with a little shopping around, but we're nearly 50 miles from any major town, so have to factor in fuel costs and time.