Here are some reputable firms offering Professional Indemnity Insurance for Yacht Brokers.
https://www.hiscox.co.uk/business-insurance/pii/
https://curtismarine.co.uk/boat-insurance/marine-trade-insurance/yacht-brokers-insurance/
https://www.itic-insure.com/our-cover/marine/yacht-services/
https://www.boatshedsupport.com/article/31-professional-indemnity-insurance
https://www.winterandcomarine.com/professionalindemnity
https://www.aon.com/attachments/risk-services/Marine-Professional-Indemnity.pdf
https://www.everardinsurance.com/
https://www.nautilusinsurance.com.au/wp-content/files/Yacht-Brokers-Liability-Policy-Wording-AIG010316YBL_0_0.pdf
https://www.globalmarineinsurance.com/yacht-brokers-insurance/
https://piibrokers.co.uk/professions/miscellaneous.html
https://falconinsurance.sc/marine-professional-indemnity/
https://1uk.net/buy/professional-indemnity-insurance
https://www.blackfriarsinsurance.gi/yacht-brokers-professional-indemnity-insurance/