Each UK JW donates £14.59 (US $18.85) EVERY week to the WT
Darkspilver, the websites you link to are summaries of WTBTS and IBSA financials. I downloaded pdfs of the actual financials last year. (I tried to attach the pdf's; I don't seem able to.)
In the original accounts, there is no separation out of what was donated from surpluses held by congregations, vs what was donated by individuals. There is a massive increase in cash coming in from 2013 to 2015, and I just don't believe there has been a massive increase in individual donations over that time.
I should point out that you can't just add WTBTS and IBSA donations together as you did in your calc, as a significant proportion of that money is in the form of "grants", possibly to each other.
If you just look at cash donations, then WTBTS received £41 million (up from £30 million in 2014 and £8 million in 2013), and IBSA received £14.5 million, (up from £10.6 million in 2014; I don't have 2013 figure).
(41 + 14.5) million / 138,261 / 52 = £7.72 per week.
However, the figure may be lower, for reasons stated earlier.
I downloaded the all the UK 2015 accounts from a link Barbara Anderson put up in around August last year. I don't know if it is still available.