Well the limited mileage would seem a question of how do they know which is which ? In the extreme case. You have 3,000 mile policy but have done 93,000 miles. Yes but 90,000 were off road and and you don't need to worry about. How does the insurance company know it was off road ? Having had all the fun changing from third party to fully comp I expect they have had more than one instance of cars not insured for off road/ play days/ foreign travel and magically becoming involved in accidents / damage the very instant they get back on insured tarmac.
In terms of 3rd party -> Comp I understood some people had third party, smashed their car up, changed to fully comp, put it in the garage for 3 months and then claimed to the insures with "OMG you will never guess what just happened...."
I suspect something similar may go on in via some dishonest types cases like this too meaning everybody is then viewed with suspicion.