I saw a tart a few years ago that charged £95 p/h and didnt feel it was appropriate to ask for a measley £5 in change.
Just before xmas i took my regular a bottle of vodka as a gift but we get on well and it was more of a personal gesture rather than a service tip.
I have had this experince in the past.
I make sure, that I have a spare 5 pounds now, so I pay the right amount, instead of 5 pounds more.
I very much doubt, she would let you off with 5 pounds less.
The RBS banks, cashpoints have 5 pound notes in them.