This rifle looks to me like it could be a Paul Roberts Rigby.

Some Roberts Rigby's did feature a quarter rib with spear point.
The "FOR BIG GAME" engraved on the receiver is also seen on Roberts Rigby's, as were swept or bent bolt handles.

The proofmarks show the rifle to have been proofed at London post 1954.
The "LP" dating stamp wasn't used until 1971, suggesting the rifle was proofed post 1971, unless a reproof was carried out.
The actual date of proofing is located under the "LP" stamping, but I cannot make out what it is from the photos.

Roberts had the Rigby name from 1984 up untill 1997, so if the "LP" date is somewhere between those years, I'd say the rifle could be a Paul Roberts Rigby.

Edited to add.............
I think the Tons per Square Inch proofmark went metric about 1989, and was replaced by "BAR".
In theory, going on what can be seen of the proofmarks in the photo, this would put the rifle somewhere between 1971 and 1990.