I mentioned this before in my Chris Oatley/Noah Bradley post, but it seems like a useful and simple way to understand finances when you're a freelancer. I'm terrible with numbers, so I sort of dread the day when I have to manage and plan my income and outgoings, but Noah's advice seems a good starting point. (Although it doesn't really convert well into GBP) He says that understanding your money in this way makes it a lot easier to understand your work's worth - a $700 (£415) is a week's worth of work, $3000(£1700) a month, and so on. It also makes it easy to split your income to your various outgoings - tax, savings, food, insurance, etc. $100 a day is the base-level income, nothing glamorous, but it is a good minimum to aim for when you're working as a freelancer.
It seems Noah has a lot of great advice on freelancing available through his online class, The Art of Freelancing. He also generally has a lot of great articles and advice on his blog. Definitely worth going back to.