It doesn't add up properly for that.

If chips are £2.20 and cheesy chips are £3, then cheese is 80p. Then if chips, cheese and gravy/curry is £3.50, then gravy or curry is 50p... except if you just get chips and gravy, it's £2.50, so actually gravy/curry is only 30p, unless you combine it with cheese, in which case it's more expensive.

Perhaps the main reason is that they can stop someone asking for chips, gravy and curry sauce, which is a crime.