If someone is going to allow you to use a chainsaw on their land, then you need a certificate and they need public liability insurance, because on their land, they have a duty of care for you (even if they’re not employing you). If you hurt yourself, they could be taken to court under the Health & Safety at Work Act. The same applies if you allow someone to use a chainsaw without a certificate on your land – if they have an accident, they could take you to court for allowing them to hurt themselves. If someone wants to use a chainsaw on your land, they need a certificate and you need public liability insurance.
If you’re cutting firewood on Forestry Commission land for example, they commonly ask for insurance that will cover you for up to £5 million.
You also need public liability insurance if you’re working close to a road on your own land – if you’re felling a tree for example, or cutting off branches – in case something falls on a passer-by and hurts them.