Seems sensible.
I mean, maybe not actually a full-on shelter (are they just assembling things in the cellar, basically?), but generally almost everyone across Scandinavia and Eastern Europe has been told, and will have about a week's supplies, and know how to take shelter. Streets have signs on where the shelters are etc (tunnels, tube stations etc used for that purpose).
Mum's all set and if that gives her peace of mind, I'm pleased for her.
A UK military base is a very legit target.
We just haven't had war in Western Europe for so long that we all think it's insanity but looks at all the countries amassing artillery and generally gearing up for war, regular Russian cyber attacks, transport networks being taken down etc.
Even if they were to lose power and water for a few days, why not have that.
Everyone did this in Covid, it might be more relevant now.
If they're happy, hey, good for them.