The Edwardian Bonehill Britannia bullpup design made every other spring piston air rifle before or since, obsolete. Everything else since is inefficent and retrogressive !
Unfortunately the piston and spring travel on an angle compared to the barrel. This negatively affects both visible follow through and recoil management. These effects become more pronounced as you increase power. Although described as a bullpup due to the placement of the piston, having the loading port equal to or in front of the trigger would not meet the current expectations of a bullpup. They were undeniably beautifully crafted and are highly prized today.