I’m with Co-op, although I probably won’t be for much longer. Don’t know what they’re like for bikes specifically, but the overall experience of making a claim with them was godawful.
We had a reasonably pricey laptop nicked last year. Getting it replaced was like pulling teeth, it took flipping ages, they completely screwed it up (twice) when passing it over to their similarly incompetent third-party fulfilment people, who would only do PC World/Currys vouchers, and once the excess was taken off it wasn’t worth the bother. By the time this year’s fantastically inflated premium came in I wished I’d just taken the hit myself. Only stayed with them because I don’t have time to shop around right now.