I am not a plumber but have done a lot of plumbing for things like this in the past and there is hardly any skill involved.
Leaving aside your slightly quirky definition of the word "skill", it doesn't matter whether or not any of it is requred on this job. If you engage a tradesperson then you do it on their terms and, within reason, pay their going rate.
I would have thought a £50 call out charge would be enough.
Enough for what? How long do you think the job would take? What overheads do you think need to be covered?