Whilst costs may be a factor I am not sure this is the primary driver.
With Warwick, I get the feeling that time was more of an issue. The facilities are much bigger than what is happening in the UK now and I don't think they wanted the lack of a new home to prevent the sale of the NY portfolio. The use of contractors has helped ensure that the facilities are ready in time for them to move out and realise the capital invested in the NY bricks and mortar. Relying on volunteer labour for a project of the size and timescales of Warwick would have meant a very elongated process.
Of course they will want to sell and move out of London as well however they are already giving it a four year time line which is a really long time to build a warehouse, some offices and a glorified Travel Lodge. They don't need to use contractors as it is likely they can get the needed resources from the congregations. They save money, get a longer time frame to beg for money and have time to get the right deal for the London portfolio. Win-win for them.