Bookkeeping is now a regulated profession and all Bookkeepers have to belong to a regulated body!! You can become a member with the ICB, but to trade you have to have their practising certificate and to get that you have to have insurance!

or

You can register directly with HMRC, they don't dictate that you have insurance but their fees are more expensive.

Even if you're doing someones books for free, under the money laundering Regulations you have to register as a bookkeeper.