Yeah, basically you form what they call a "captive". A captive is your own insurance company. You need pro advice to help to create it, and then you need to get experience insurance claims and underwriting staff to administer it and pay third party claims (or you sub-contract it out to a Broker). All kinds of things are needed - a full directory of motor engineers for a start.
You can issue your own certificates of motor insurance, legally.
Some clients prefer more help and ask an Insurer to "front" the captive for them - the client uses the insurers administration structure, simply plop £250,000 into a joint account (client and the insurer) and they dip into it to pay claims made against you. The Insurer will charge a fee for the use of their services - anything from sort of £50k upwards depending on the work involved.