The Watchtower Society sits on millions but has a callous disregard for the millions of people it keeps impoversihed. It could self fund the buildiing but they would prefer to take more money from the congregations.
They have to keep Mill Hill running and printing whilst they build they will then transfer the equipment across and then sell the London buildings. The build will need to be funded first. The society could take out a short term loan but why should they when there are people willing to throw money at them no questions asked. As they are only paying for materials not labour the new facilities will be built for a fraction of the cost of any commercial build.
The sickening thing is that they are not asking to borrow from the congregations they are taking from them and when the London properties are sold they will not be giving anything back they will be rubbing their hands and making pure profit.