I think, in this condition, it would be £100 - £120 max. The buyer would have to obtain the missing pieces to restore it. It might be that the spring and piston also require replacement, such is the condition on the outside.

I would contemplate buying a 45 in good condition, in .22, to add to my existing .177 but nowhere near in this condition. It might be better to keep it and restore it in due course, to obtain a better price. These were good rifles in their day.

Rgds
A