Elephant hawkmoth caterpillar. If it seems late, it's because they pupate over winter and emerge in the Spring. If they're common where you are, you're very lucky!
Where and when did you find it? If you still have it, it may be worth keeping it and letting it pupate. Although if you found it in the day, it's probably parasitised. Still, I'd keep it and see what happens to it! It eats willow-herbs and bedstraws.
Truth be told, I been here a while, I just never post. I mostly watch the Myriapod and Scorp boards.
Thanks for the info It was found yesterday by my brother in Chatham, Kent. They must be pretty common, as his cat keeps bringing them in. Lol. I'm in Portsmouth, so I'm not in a position to keep it. Though, I will pass your information on, and if he decides it's interesting enough to keep, I'll get pics and update this thread. Thank you again