For accountants, professional indemnity insurance isn't legally required – but if you're licensed by ICAEW, ACCA, AAT, ICAS or CIMA, it's a condition of practising, with formulas that dictate your minimum cover.
Know your body's formula, insure at or above it on compliant wording, and disclose your work split honestly – tax work drives 70% of claims and underwriters rate accordingly. Add tax-investigation cover (clients increasingly expect you to have it) and plan run-off before you need it: ICAEW wants two years, ACCA six.