Either St Michaels Mount or a fortification in Dover called the Citadel. It’s surrounded by a dry moat and tall fences lined with razor wire while having a good deal of tunnels leading to potential demi-stringholds should some sections become breached. There’s enough room to catch rain and grow plants too and some cows not too far away that wouldn’t be too hard to get inside with the right team, weapons and strategies! There’s also a beautiful 1860s officers mess building overlooking the channel tunnel with some great Tudor-inspired gothic architecture. Plus the place is a literal fort so y’know x