I can see why it's a bit sad to not see your traditions passed on - but that's something to talk about with your DIL, not presume all English Christmas tradition are disappearing. Plenty still go on.
It's also sad that your nanna enforced her traditions and perspective with violence, or at least the threat of hitting you, especially over something so slight.
I was raised with both violence and threats of it over the idea of how Christmas 'should' be, how it was perceived mattering more than how it felt. It felt like a performance, one I stopped participating in at all as a teenager.
My in-laws were fine with this until my DH and I started having kids. We had a rough few years until we could get past our own defensiveness and talk about how we could bring our ideas of what we wanted to share with the kids together.
I still think Father Christmas would find milk that's been sat out for 4 hours and slightly tepid revolting and would surely cause travel sickness.
Quite possible, I agree it doesn't sound nice, but drinking tepid brandy - or any other alcohol - that's been sitting out for 4 hours isn't great sounding either, and even drunk 'fresh', alcohol is far more likely to lead to hypothermia if we presume the drinks impact him and not neutralised by ~magic~.