I have created an overview of most of the figures of the IBSA and WTBTS for myself. The relation is quite complex. WTBTS produces the literature and hires accomodation (and people I guess) from IBSA to do that. Then it sells that literature to IBSA (for distribution I guess) at production costs. But the IBSA reports mention that all costs it makes are reimbursed by the WTBTS. Maybe both are true, but it's fuzzy to me.
Here is what I noticed, this is 2006-2013:
- donations are quite stable, per year ~15M pounds in cash, and ~3M pounds in legacies (by testament after someone dies I think?), with a peak in 2013 (5% up), but there was also a peak in 2007 (30% up), so I'm guessing these are outliers
- someone donated ~5M pounds as property in 2011 to the IBSA
- literature producing costs are stable
- there has been a big inflow of money from the German branch from 2011 onwards
- there has been a big increase of grants/overseas support particularly in 2012, but also 2011 and 2013 (I guess for kingdom hall building), the German money seems to have been used to support Africa and other continents with building halls and other local congregation needs there
- very little is being used for humanitarian aid (that's not news though!); e.g. 2011 reports grants of ~20M to support overseas missionary work, but only 110 thousand to support Haiti shelter relief buildings + kingdom halls (of course!)
- convention costs were quite stable, but significantly more expensive in 2012 (60%) and also, but less so, in 2013 (40% compared to 2011); OTOH these costs are 5% of the total or so
- I think New York coordinates shuffling money around to keep the balances in check; every year's report mentions other countries donated to ranging from Brooklyn HQ to Europe to African bethels.
Anyway, mostly looks to me like "business as usual". I can't see how they can finance ever-expanding bethels (and accomodating people there) though, the funds seem to be stagnating, not increasing. Although with these cross-country "injections" (grants and overseas donations) it's hard to draw definite conclusions from the London branch alone.