It was for the Prudential. Also featured a kid saying "I want to be a tree", a phrase often repeated in my school playground in the same annoying twee voice! I hated that ad.
It was a grown man, hippie sort, naked to the waist who said "I want to be a tree", not a kid. The ad also featured the guy who plays Max in Eastenders, with a punk haircut, saying "I want to be a brain surgeon".
The prudential advert stared Mark Williams and Jo Unwin - You can watch the advert on YouTube - https://www.youtube.com/watch?v=n3hStlMYE2k - It is the 4th advert which stats at 1min 26secs