First of all, you would have to set up a business like this through a limited company to limit your personal liability in the event of a claim which turns out not to be covered by insurance.
Have you checked the websites of other bike tour companies? What disclaimers do customers have to give? I suspect that the probably charge for insurance or build it into the cost.
I would think that you need two types of insurance - public liability for damage done to a member of the public by one of your cyclists. This would be cheap enough. But you will also require professional indemnity to indemnify yourself against you own negligence - e.g. faulty brakes, failure to supervise, etc. This would be a lot more expensive.
Insurance brokers can be useful in such situations, but I would not be relying on them for legal advice. However, sometimes there is an insurance company which specialises in this type of narrow area, and they might well know the legal liability well.
As part of your market research, you should probably go on a few tours in the UK with different companies and then ask them what their liability and insurance situation is. Their legal system is fairly similar to ours.