Sadly it's very unlikely that they will have any real value (probably less than the face value of the stamps). This topic comes up from time to time on MoneySaving Expert - for example:
https://forums.moneysavingexpert.com...rst-day-covers
https://forums.moneysavingexpert.com...rst-day-covers
https://forums.moneysavingexpert.com...mment_79366477
https://forums.moneysavingexpert.com...mment_78552146