The government/MOD don’t carry insurance, they’re big enough to cover the risk themselves rather than pay someone else a premium to do so. (Link 1)
Sort of like how, if you had enough money, you didn’t have to pay for car insurance, just have a deposit with the Supreme Court… (Link 2 - section 144(1))*
https://assets.publishing.service.go...5_redacted.pdf
https://www.legislation.gov.uk/ukpga...-risks/enacted
* That loophole was repealed by means of a Statutory Instrument in 2019)