We bounced around assorted insurance companies over the years for our two cars. Things got a little muddled between named keepers, owners, primary drivers and the like so we did a consolidation in order to get on the correct side of the small print.
What seemed universal and also echoed my previous trades experiences was the reg keeper/primary driver/other cars use extension.
DOC is far and away only for named policy holder only and for third party only. Further, the other car must have a stand alone insurance policy in force and not reliant on the DOC driver, thus preventing an uninsured car magically becoming insured because your Harris is in the drivers seat.
In the main, the majority of scrotes I dealt with back in the day relied on this as a get out of jail card. Watching more current interceptors type telly shows that it’s still a very common misunderstanding. It is quite expensive and so, rare, to get 3rd party cover on current fully comps. If you do, at no extra cost, then we’ll done.