Mortgage brokers are FCA-regulated, and that changes the insurance conversation entirely: professional indemnity isn't advisable, it's a condition of authorisation, with minimum limits written into the FCA Handbook (MIPRU 3).
Compliance is the floor: MIPRU limits, regulated excess, unbroken cover since authorisation. Above the floor, buy against your product mix – later-life and equity-release work needs limits and wordings mainstream books don't – and treat run-off as part of your exit plan's cost base. Use a financial-services PI specialist; this is not a generalist's market.