Alasdair Taylor's Answer

There is no general rule of English law that a company must carry insurance with respect to customer claims, or that they must disclose details of that insurance to a customer upon request.

However, for some particular types of business, there is an obligation to carry insurance (e.g. solictors).

I don’t do any litigation work myself, and I am unsure whether any of the Pre-Action Protocols require this type of information to be provided.  I suggest you have a look at the Protocols, avaliable here:

https://www.justice.gov.uk/courts/procedure-rules/civil/protocol