Big disclaimer: I’m not a bookkeeper, but I’m sure the ones on here will keep me right!! (Guys - jump in if I get stuff wrong!)
Firstly because bookkeeping has a legal element, you either need to be certified by one of the governing bodies (ICB, ACCA etc) or register for Anti Money Laundering Regulations with HMRC. My understanding on the qualification by experience route is that it’s blooming difficult and you have to pass various exams. However registration with the governing bodies mean that they rubber stamp your registration, and also that you get access to various training and networking events through the year… The registration fees (I believe) are roughly equal to what you’d pay for MLR registration.
NB: You cannot advertise bookkeeping or perform bookkeeping functions without registration or licensing in place. HMRC define bookkeeping as being something which affects the recording of accounts, which means that if you load stuff onto a live system like Xero/Kashflow/Sage Online/Quickbooks online etc, you are performing bookkeeping/accountancy.
HMRC in 2018 in conjunction with SVA and VAProMag launched an exemption for virtual assistants who perform limited bookkeeping as part of their business - but the rules are you must be turning over under £30k, the bookkeeping function must be less than 5% of your turnover and you have to register with them… Various other bits n pieces, but you can read more here: https://www.societyofvirtualassistants.co.uk/2019/04/24/anti-money-laundering-regulations-for-virtual-assistants/