Bookkeeping is the gentle end of the accountancy risk spectrum – but "gentle" isn't "risk-free," and if you're licensed by AAT, ICB or IAB, professional indemnity is a membership condition with defined minimums.
Buy PI at your body's formula (or £100k+ if unregulated), on an any-one-claim basis, and be honest about scope creep into tax work. Then take the cyber question seriously – for a modern bookkeeping practice, the likeliest bad day isn't a negligence claim but a compromised inbox and a client's redirected payment.