Most property insurance policies include an Average Clause (or The Condition of Average).
To explain this simply:
If you insure your property for £500,000 but the actual reinstatement value is £1,000,000, you are 50% underinsured.
As a result, any claim made under your policy (whether it's for a partial or full loss) will be reduced by 50%.
For instance, if you file a claim for Accidental Damage amounting to £10,000, the insurer will only pay out £5,000.
RebuildCostASSESSMENT has also created a short video that elaborates on this topic, which you can view at the bottom of this webpage.