I'm sure this used to be the case, but it seems to have changed:
"Basic compensation for actual loss: Up to £50 for the Royal Mail Special Delivery Guaranteed by 9am service and £500 for the Royal Mail Special Delivery Guaranteed by 1pm service or the market value whichever is the lower and the fee repaid.
Enhanced compensation up to £2500 is available if purchased at the time of posting."
—Making a claim for a lost item
So you'll get the amount insured even if the actual value is higher.