How much do menial jobs like washing plates, cleaning and so on pay per hour in UK?

The current minimum wage is 5.35 pouns per hour, so you can expect prehaps a little over that. If you work at the wage stated for a long perid of time (in excess of a year) you can expect to be taxed and lose 10-20% of your annual figure, all depending on what you work as, where, and who you are.

Most of the time though you wont be able to find a occupation that will give you the same menial job to do all day. You mention washing plates, for example: Your work place becomes a kitchen, its unlikely they will have plate for you to wash all day, so you will most likely become a "handy man" about the place, washing plates here, mopping floors there, cleaning tables ect. as the need appears.

Hope that helped.