Take to the old ways and find high, defensible ground - possibly an old castle, fort or fortified manor house. It should already have a walled garden for produce, productive lands and coppice, carp-lakes or fishing or access to the sea and the technology in the house should be old enough that you could rely on it as it was pre-industrial revolution.
I have my national trust handbook and would not be afraid to use it!!!!
Hands off these ARE MINE!
http://www.nationaltrust.org.uk/main/w-vh/w-visits/w-findaplace/w-lindisfarnecastle/
http://www.nationaltrust.org.uk/main/w-vh/w-visits/w-findaplace/w-powiscastle_garden/
http://www.nationaltrust.org.uk/main/w-vh/w-visits/w-findaplace/w-tattershallcastle/
Warwick castle looks good.
Part of the problem, would be that they are not complete any more.
I would be tempted to go for one of the forts in Scotland.
Fort George because it is designed for modern weapons, and is a military base as well.
There are at least three others of this design : -
Fort Cumberland
Tilbury Fort
I think Berwick Barracks is one as well.
I would be more tempted by this group, the HHA, as they are all owned, and mainted by their owners. Like Warwick, but I would be very tempted by
Bamburgh Castle. It is an amazing location, and not sure if it still does, but had its own electricty supply. One of the first places to get it.
This has always been my idea, but you do wonder if some more modern building might not be better.
Clear a mile round a big block of flats, factory, or similar many floored building. Basically make yourself a big vietnam era fire base. A motor factory, with square foot gardening on a large scale.
You need something over 120 people for a stable gene pool, and the human race to survive. How that breaks down, I am not sure, but you would have to be very careful. Also what effect families etc have on this, not sure.