Take your target salary and add any non bookable overhead costs, e.g. rent, heating, t&s, professional fees such as accountancy and legal, pensions, health insurance, institutional fees, training, liability, printer cartridges etc.
Take 52 weeks, take 5 weeks off for holiday, 8 days for bank holidays, at least a week for sickness.
Take off any time you spend doing non chargeable admin, answering emails, your time connected with overheads above.
Take a deep breath, sit down, pour a drink, and divide one by the other.
Et voilà!
(I reckon you're looking at £60ph pretty quickly)