The London Mathematical Society is a small but important institution which operates out of De Morgan House in London. It publishes a few (very high quality) books and journals, organises and supports conferences and symposia, and has small grants to give out for mathematical activities. The LMS also bestows highly regarded prizes and medals for mathematical research. Its focus is research into pure maths.
There is a firm plan to merge the LMS with the Institute of Mathematics and its Applications (IMA), no doubt a fine institution, but one about which I must plead ignorance. Its focus is applied – or applicable – areas of maths.
There have been meetings around the country to discuss the proposed New Unified Mathematical Society, with the Presidents of both societies present (but me absent).
I can’t say that I have weighed the arguments carefully myself. But certainly several mathematicians are deeply concerned about this plan. If you have a view, you can follow the debate at their Save the LMS blog.
Simple City 
Like you, I haven’t given the matter much serious thought; but something in the tone of this newsletter raised my hackles slightly. For instance
A New Mathematical Society has better prospects of doing this because it will have the organisation and scale to set the mathematical agenda rather than always being reactive to threats.
[Emphasis mine.] I remain to be convinced that any meaningful agenda — except “Give us more money!” — can be applied across all sub-disciplines and cultures. Who sets the agenda?
(BTW, consider this a belated attempt to fulfil condition 5 of this meme, which you should of course feel free to ignore.)