As long as they haven't been eating privet, you can feed them to your chameleons. They need to be raised on an artificial diet (which can be purchased easily in the UK/Europe, from what I have seen.)
They are kind of like the UK equivalent of a hornworm since they are so big and need to eat an artificial diet. If they were easy to buy here in the US, I would buy them/breed them. They are very easy to breed and keep (I've had them before, but I couldn't find a source for an artificial diet and didn't have the resources to make one at the time so I stopped breeding them, even though the moths are giant and beautiful.) They would be great to add variety and to add a nice large worm for them to eat.