I think pocket money should be a token amount. While I like the idea of losing 10p for very naughty behaviour, I agree it should not be a payment for chores or good results.
A quiet trip to the cinema or toy shop can be a reward for doing well, and it keeps the value of special family time.
It's not easy to know how much to give, I would err on the side of less and increase it a little annually to reflect 1.inflation and 2.the age of the child.
Perhaps when the child is older, say 10 years, you can introduce a savings scheme where they are rewarded with a few £ extra for saving up for something they really want.
Good luck striking the balance x