That was fun, though much less testing than recent Prizes. My partner does the 12a in our household (definitely not 11a!) but I have heard of, or could guess, most of the specialist terms such as 11a L. (Sympathies to the sensibilities of those who sought web guidance without due care!)
FOI 11a, LOI 6d, COD 18d for its double theme (I realised after reading the comments that, although the 18d phrase was familiar, I know it from campanology via Dorothy L Sayers rather than any 12a context). 3d a lovely runner-up COD
I didn't know of the theme event. I have witnessed a couple of times the 20a 11a bike ride, which periodically adds to the gaiety of inner city life and might appeal to some of the same participants (or others inspired by this puzzle to join in)!