Land and property prices, particularly in London, have escalated in recent years. If there is a post-Brexit price crash looming in the next few years (as seems reasonable), it could be very good business for them to sell now. There might even be a possibility of buying later for a reduced price.
What's happening with the KH's may be more about a reaction to property prices than anything going on within the organisation.
The move from Mill Hill to Essex is about comparative property prices. I imagine that the exodus from New York City is along those lines too.