NATO are just helping out their British chums by suggesting closing Northwood. They have been watching the determined way SDSR and subsequent cuts announcements has shown Britain's desire to move to a third class defence force - so they don't see why NATO assets should be based here.
Or it could just be that we have 3 times as many HQs as required in NATO, and that unfortunately 2 of the 3 (e.g. Lisbon, Naples and Northwood) will need to close. Naples appears to have got a stay of execution following the Libyan crisis, so its goodbye to Northwood.