I keep a few crates of water in glass bottles in the cellar. Have enough food. If I lived in a flat would stash a few five liter containers (stack well) under the sink.
An attack along the lines of WW3 would be so catastrophic it almost isn’t possible to prepare. More worried about a nuclear power plant being inadvertently hit in Ukraine or critical infrastructure disruption.
I think people with gardens/local communities/schooös should be encouraged to grow more. Particularly fruit trees and berries. The UK is ridiculously reliant on imports. Plus you can easily grow berries and fruit that is organic. Also cuts down food miles and packaging. Well kept chickens too.
Solar charger and some solar lights a good idea too. Can easily be part of a camping set.