This has only just gone on the market and it clearly says that they're looking for a rent "in excess" of £50,000 per annum.
I'm still surprised why this charity you say you know would even be considering something so wildly expensive. It's not like the space is ready to move into, or it's even particularly suitable for training.
What do the other three businesses you say are interested in the underground loos do? It's really quite an awful space for the price and unless they have an overwhelming need to be right in the middle of a public space in the heart of Brixton, I can't see why anyone would want to commit to such a pricey, dingy and space-limited venture.