The Road Traffic Act 1988 (Part VI Third-Party Liabilities, section 144) states

Section 143 of this Act does not apply to a vehicle owned by a person who has deposited and keeps deposited with the Accountant General of the Supreme Court the sum of £15,000, at a time when the vehicle is being driven under the owner’s control.

The Road Traffic Act 1991 updated this amount to be £500,000.

The laypersons translation of this, of course, is that you can drive without car insurance in the UK provided you deposit £500,000 with the “Supreme Court”.