*** Further Pedant Warning ***
I don’t know if your answer quite made it clear, flip_flop, that the upper limit (which is £1m) applies to damage to property only. There is no requirement in law to have cover above that amount for that risk. Here’s an extract from the Act:
The policy shall not ...be required—
(a) [Irrelevant]
(b) to provide insurance of more than £1,000,000 in respect of all such liabilities as may be incurred in respect of damage to property caused by, or arising out of, any one accident involving the vehicle
However there is no upper limit on cover required for causing death or personal injury. As you say, most insurers usually provide unlimited cover for both risks.
Until 1988 (I think the change was made with the introduction of the 1988 Road Traffic Act) there was no requirement to have cover for damage to property at all. Cover was only needed against death or personal injury.
However, we digress!