Forgive me for asking but how will they know, or how will anybody find out you are volunteering?
Well, I guess they won't necessarily know, but if you are found out, there can be serious consequences. We have seen people be refused entry to the UK in the past because the IO has discovered they did something as seemingly innocent as babysitting for a friend on a previous visit to the UK.
In the case of illegal working or volunteering, the worker/volunteer can be arrested, and possibly removed from the UK and banned for 10 years, or if not banned, it can seriously affect their ability to get another visa in future.
If the employer is found to be hiring illegal workers, even volunteers, they will be fined £10,000.