You should tell your insurer before any heating work starts. Most policies require notification of significant building alterations, and a heating installation counts. Failing to notify could affect your cover.
In practice, moving from a combustion system to an all-electric one may improve your risk profile because you are removing a source of fire. Some insurers offer lower premiums for buildings with no gas supply. Conversely, installing a biomass boiler introduces a fuel store and combustion risk that your insurer will want to know about.
Ask your insurer what documentation they need. A certificate of installation from a qualified engineer and evidence of annual servicing are usually sufficient.