As someone who has done some 60k miles on 2 wheels, I think flying is vastly safer. There are far too many "blind" people on the roads today, and if somebody pulls out of a sideroad, you have no chance - unless you ride so ridiculously defensively there isn't any point in having a motorbike. In flying, perhaps 99% of the risk is under your own control and there is a helluva lot an individual pilot can do.
And en-route flight is particularly safe. I think that flying say 300 miles is safer than driving that distance in a car, especially if it is country lanes. Unless, again, one drives unreasonably defensively.
Re insurance premiums, most likely the number of people killed or injured in GA is too small for an actuary to be able to work out the risk accurately. 3rd party (ground) damage data is even harder to come by. On top of that, the underwriters are all anonymous Lloyds syndicates (BAIG aside) and without any public relations profile they don't need to offer good value.
Finally, regarding competition, if anyone thinks there is competition, they should contact say ten brokers for a quote on the same plane, and see what happens... what happens is that nobody will want your business because the underwriters don't like people that shop around.
I've had some dealings with Lloyds in other areas (product liability) and been told that on the more obscure types of cover where there is very little data they often work out their premiums by throwing a dart.