I think £40/h £200/day for a plumber is about right.
I'm just south of Leicester and charge 60 for first hour and 40 p/h after, 300 for day, 100 for out of hours callout.
When I first went self employed I started at 50 first hour, 35ph after
Looking at changing to 60/h with 20min increments after first hour as I tend to undercharge for jobs that go over the hour.
350/day, 200 for 4h
I do provide a good after sales service and free quotes, if the job is very straight forward and wasnt out of my way I'll sometimes take £20 or £40. It's surprising sometimes how little I can make in a day, like today I was out for 11hours and only made £160, with my weekly overheads at £200 that doesn't pay the bills!
I have to take at least £1000/week to make being self employed worthwhile