You want specifics? Here are specifics.
400bhp's view is that the risk premium of insuring cyclists is so small (demonstrably so because the CTC and BC both offer insurance more-or-less for free as a part of membership) that compulsory insurance would be cheap because of competition. I suspect that compulsory insurance for bikes would become more like PPI cover than like motor cover - prices will go up because access to markets will be controlled by intermediaries and because admin costs would be astronomical relative to the risk premium. Very few insurance companies would be interested in writing the pence per policy that this cover would attract. Those that are would be happy to outsource most of the administration, and most of the profit, to an intermediary. Much of the premium would go in the frictional costs of marketing rather than to actually provide the cover.
I wouldn't call some of that as much specifics as I already mentioned admin charges, of which 400bhp objected and one can logically draw this conclusion highlighted. That is fine, 400bhp didn't say that though and it is a great improvement. The marketing is a reasonable point, I tend to imagine not much marketing going on in any cycling activities for much the same reason we have with cycling at the moment where a lot of people who are new to cycling have heard of halfords (who do obviously advertise) or take the word of someone they know for buying stuff.

However twowheelsgood and your later post are also useful.
However