Fox is correct from a permanent employee but this isn't what the OP is talking about. He's talking about 20p/h freelancing which is easily achievable. Pro-rating it up is silly.
yes this is what I see and I think most bookkeepers will freelance. I cant foresee bookkeeping for 1 company all year.
Plus some companies only require a few hours work per week...so you can be doing multiple books at once.