If you have any employees, you may be legally obliged to hold Employers Liability insurance with a minimum of £5 million indemnity cover. See Employers Liability for more information about whether this applies to you. The reason for this is to ensure the well-being of your employees should they get injured or become ill through work.
Whilst other business insurance policies aren't required by law, there may be stipulations in your contract that make policies such as Professional Indemnity and Public Liability compulsory, in which case there may be a contractual obligation to hold such policies, or in the case of Professional Indemnity, it could be a requirement of a professional body as is often the case for accountants and solicitors.
In terms of the IR35 legislation for limited companies, it can also be detrimental in an enquiry if your contract stipulates business insurance but you don’t hold it as this demonstrates that the contract is not a true reflection of your circumstances.
Beyond this, your choices to hold business insurance should be based on an assessment of your risk and appetite to cover such risks.
Public Liability insurance is the most commonly held policy for the self-employed, particularly those in manual work, as most business owners accept that there is a general risk of a third party claiming injury or damage to property, whilst those offering professional advice such as bookkeepers will commonly hold Professional Indemnity as these individuals accept that the nature of their services opens them up to claims of financial loss.
Other popular policies such as Jury Service & Legal Protection, Contractor Sickness Cover, and Tax Enquiry Insurance are held when contractors and freelancers wish to cover these additional risks of running a self-employed business such as the financial loss in the event you were called for Jury Service, the consequences of a long-term illness, or the ability to defend a tax dispute.