I think that is the main problem if you charge low, the customer thinks that you need the money and you will accept their crumbs from the plate.
What you should do is raise your price to £10 minimum with double on a 1st clean.
You then increase the price to £12 if they want it cleaned every 6 to 8 weeks, anything over this pattern is charged as a 1st clean.
I have always dropped customers who I didn't like or want from day 1, yes it can be tough, but replacing them is easier to more confident you get.
Remember it's your business and your customers need you, customers can be replaced if they are not good at their job.