Webley service Mk 2 .177

[Troubledshooter]

The catch under the barrel you refer to, is known as the interceptor sear or 'Anti Bear-Trap device' and is a safety feature that should prevent the rifle being accidentally fired until the barrel is secured in the breech housing.
It should be spring tensioned at the rear so that the front end engages the piston when the barrel is in other than the closed position, and should release when the barrel is lowered and engages the back end, thus releasing the front end from the piston.
If you lift the barrel away from the breech housing without actually cocking the rifle, and remove the barrel, you should be able to move the interceptor sear quite easily against it's spring, as it pivots on a pin that is a friction fit between the two lugs on the cylinder.