The proximity to London is quite good (hospitals there and all), but the other ideas aren't at all at play (wwii, "solidarity" with the people, or what have you).
They are simply very set in their habits, that's all. Easter is always spent at Windsor, so Phil and Liz are isolating there. Next week they'd have started their Easter break at Windsor, which was now moved up a week. If they'd spend Easter every single year since the beginning of mankind at Balmoral, they'd be at Balmoral now, or whichever other estate they have.
Liz lives more at Windsor C these days than at Buck P, being at Buck P Mon - Thu (used to be longer at BP, but with age she's extended her weekend at Windsor, which she's well deserved at her age).
William isn't acting monarch now, as the monarch is very much fine, as is the monarch's direct heir. They are just simply now "working from home in isolation" so to speak.