that’s what I thought initially too… but Guidance for the Period of National Mourning following the death of His Royal Highness The Duke of Edinburgh - GOV.UK does say
A period of national mourning has commenced and will conclude on 17 April inclusive.
it seems I was confused about the difference between “Period of National Mourning”, and “Period when flags will be half-masted”, which does end the day after the funeral
Union flags flying from Royal Residences and Government Buildings were half-masted on 9 April and will remain half-masted until 08.00 on 18 April, the day after the funeral.
(I’m no protocol expert, but I’m not convinced “National Mourning” will really end before the funeral. It would make sense for it to end when the service finishes?)
(but yes, separately, there is a definite typo in the message @themajor pasted!)