It is a load of ? hogwash
If the pilot of the helicopter voluntarily flies to do a prison raid, I hardly think that a restricted area will put him off. If a helicopter is hijacked, a gun to the head of the pilot will have the same result, which more than one of our colleagues can relate. The sizes of the areas vary but it buys no more than a few seconds of theoretical warning time. This amounts to zilch in terms of additional "protection".
The remedy is obvious and cheap and in the hands of the prison authorities. Place a lattice of wires across any areas into/from which a helicopter might land in their prison - problem solved.
But they would rather load us up with useless additional restrictions on our flying which cause nothing but trouble for anyone to no useful purpose.