I have consulted WhatTheFont!, Identifont, and FindMyFont, with pretty abysmal results. I would like to identify the font in the image below: This is the cover of a pocket-sized orchestral study score, printed probably sometime between 1967 (the date of the copyright) and 1984 (date of purchase of this particular score). Boosey & Hawkes is a London publisher of study scores and other music.
The english entry for Gill Sans on Wikipedia reveals some more information on the mysterious Q. It seems that both variants were used in different sizes of the historic metal-type version, at least for this specific Title variant.
An American metal-type specimen sheet of "Gill Title". Note the original "5", "7", "0", and "Q" in some sizes, which were dropped in many later metal type issues and digital versions, and a non-descending "J".
It's Gill Sans with an alternative 'Q'. The current Gill Sans Nova digitisation does include it.
That style of 'Q' was very normal in the nineteenth century, like on Century Schoolbook, and the metal type version of Gill Sans included it (and many other alternate characters) which were available by special order. Here it is in the introductory specimen along with an alternate R and J as well, but that sample still does show the normal Q we're used to as well. It might have been done to give printers a 'Q' that doesn't drop below the baseline at all if they wanted to give lines of capitals a more linear effect, like with the alternative 'J'.