You’ll possibly need two things:
1. To have your motor insurance insurer notified that you now use your motorcycle for business use, the ‘business’ being guiding people on your tours. Depending on what exactly you are doing, this might well be possible for free. For example, friends of mine who do on-road training for coaching bods to take a RoSPA / IAM test most often get charged nothing or very little. Some of them do this under the banner of a RoSPA or IAM affiliated group and some under their own private enterprise and some under both.
However, it might depend on how much of a ‘business’ your new enterprise is. In other words, are you doing it for:
A. Fun and zero financial reward, a bit like when I took bods on Wapping’s Wanders
B. A modest fee, simply to cover your ‘reasonable’ expenses for the day
C. A profit making enterprise
2. If it’s a true business (and maybe if it isn’t but you want protection from possibly being sued) some form of public liability insurance (beyond and separate to what is covered by your motor insurance) something a bit like this:
https://www.devittinsurance.com/training-school/
https://www.devittinsurance.com/training-school/
These might be a bit too broad as they seem to be tailored towards full-on training establishments, whilst you are ‘guiding’ people (presumably they’ll be paying you, so you’ll maybe be making a profit of some sort and you’ll have entered into a contract with your customers) but the limits of liability cover mentioned are maybe - I don’t know - not too unrealistic.
Just because they are a ‘good bloke’, enjoy a brew and a laugh and ride a motorcycle, don’t assume that one of your customers will not sue you if something untoward happens. It might not be them don’t forget but their wife / partner, should they feel that you - through your negligence - got them killed or injured. However, temper this through the realisation that:
A. You’ll probably have to be proven to be negligent to have an award made against you.
B. There is no point in anyone suing someone who has no assets or money to pay the award granted; the so called ‘man of straw’. In other words, you might need / want to run this ‘guiding’ venture through some sort of limited liability company, where your financial exposure would be limited.
If nothing else, your public liability insurance may well come in handy to indemnify you for your legal costs in repudiating the scurrilous action brought against you by that ‘nice bloke who rides a motorbike’.
PS Liability insurance for this used to be cheap but, like many other costs, has in some cases maybe been rising over the last couple of years. It’s not something I do day-to-day, so talk to a proper broker / intermediary.
PPS The ‘Green Badge’ (whatever that is) organisation might be able to help you, perhaps? They might, as some sort of ‘organisation’, carry their own liability insurance. But that is just a guess.