Blogger and Unitarian minister Andy Pakula divided church giving by the standard capitation to make a working -- if by his own estimation, highly inaccurate -- census of inland churches affiliated with the General Assembly of Unitarian and Free Christian Churches.
I think it becomes somewhat less inaccurate -- particularly to those who may have an accurate count -- if these are seen as a jumbled quantification of membership numbers, wealth, institutional capacity and warm feelings towards the general fellowship. Or at least this is based on my experience in the United States when I review Annual Program Fund giving. In particular, I would love to know how the Christian Unitarian and Free Christian churches stack up. (It seems the Welsh-language churches have problems in fact or in relationships.)
And in my scant experience in London. When Hubby and I honeymooned there in 2003, we bypassed the usual tourist church options and attended Sunday services at the Unitarian church in Brixton. A wonderful welcome, and a delightful service with a significant congregation, but now registers with zero members.
Now, for the numbers, as a mental exercize: assuming these numbers were correct, most of the 169 British churches would be ineligibly small (under 30 members) to join the Unitarian Universalist Association, and many would be too small to join under the old rules (10 members). Only one -- Rosslyn Hill Chapel in American-expat-heavy Hampstead -- is bigger than "small," or having 150 members or fewer. This makes me doubt any significant numbers of foreign ministers can be tempted to settle there, and I'd say the same if every membership value was doubled. (That said, Rosslyn Hill has a vacancy, and it has had American ministers.)
I've also made the data into a handier spreadsheet, though be sure to note corrections at the original post.