Getting insurance for bitcoin holdings is practically impossible, mostly due to the potential for massive swings in value.
So factor out the swings in value by insuring in a more stable currency.
Obtaining insurance for "up to £X worth of bitcoins per account" seems like it aught to be possible. If X is a small number, then that's a strong indication that people shouldn't be using Vibanko for secure storage of coins.
If you can't find an insurance company that will cheaply sell you (say) £1,000 of coverage for (say) 10,000 accounts then that is an indication that maybe you're riskier than you think. Can you afford to self-insure, and put up a £10,000,000 bond to cover potential losses? Or work backwards: how large a bond can you afford to put up? Divide that by the expected number of users and that's a good indication of how much wealth users should trust you with.