We use Cow and Gate (hospital had a choice of SMA or C+G and needed to put something down her NG tube - I told them to use whichever they saw fit to - so she ended up on C+G), now on the hungry baby one as we were trying to delay weaning till as close to 6 months as possible and she was a bottomless pit of hungry!
Realistically though - Christmas these days - the shops shut for one day... ok so you might need a riot shield and helmet to get THROUGH Tescos on Xmas Eve or Boxing Day but they're generally open... plus in the absolute worst-case scenario, your NHS area will have a rota of chemists open over the festive season (ours was usually published in the local paper) and 99% of them will stock formula if you end up utterly utterly stuck. (Unless you do what we did one year and get stranded in the middle of Inverness airport on December 30th with no flight home for three days... but that one's another story and we won't get into my opinion on RyanAir!)