It is not essential for any individual shooter in the UK to have insurance (although it is recommended, and is often a requirement of leases).Who are you insured with to take out your clients?
Clivertons decision aside, anybody squeezing that trigger should have their own insurance should they not?
If you're taking people out stalking, and especially if you're charging for the privilege, the onus is on you to have the necessary insurance cover, not the client.