This is the UK. A full size rifle will be better and lets face it your "Survival" air rifle will get used once for tin cans then shelved when the zombies don't arrive in december.
A full size/ carbine rifle will be of better use and will get more use.
If weight is the issue then skeletonize the stock. - maybe shorten the barrel a few inches if you so wish. A reasonable project in itself.
FEATHER FORGE - Traditional Blacksmithing
He who asks is a fool for five minutes, but he who does not ask remains a fool forever. Chinese Proverb