In Scotland there's a (pretty amazing) initiative called Business Gateway that runs free workshops on all aspects of running your own business including accounting, bookkeeping, marketing, social media and much more, as well as the possibility of one to one support. I would imagine there must be similar equivalents in other parts of the UK? Local authorities / job centres might know if there are, or job centres might offer similar training / support / courses for getting started?
(Worth noting that they are actual 'workshops' and a bit interactive so worth thinking through your cover story for worked examples etc if you don't want to declare sex work or have to come up with something on the spot!)