First, let's deal with the proof marks.
No, it's a crown over "GP" interlaced in a cypher. This is London's definitive proof mark.
London's view mark.
It isn't a winged G. It's lion rampant over the letters "GP" interlaced in a cypher. This is London's provisional proof mark, used to denote test of a barrel before the barrels are made up, and before final, definitive proof.
sideways “W” or sideways “17”
This is not a British proof mark. I don't know what it is.
The charge engraved on the flats was put there by the maker, not the proof house. British proof marks are stamped, not engraved, and were, and are, highly standardized.
Missing is the mark that indicates bore size, which would have been a "10" or "12" (4 drams and 1 1/4 oz shot). This mark may have been forward of the flats on the round, which may be why it's missing.
From the charge engraved on the flats and the proof marks, these were, originally, British shotgun barrels - a 10 or heavy 12 gauge. Since they are now rifled and are now of much small bore diameter - I assume they're now rifled since you're calling them rifle barrels - they have since been sleeved or lined. The seams between the original barrels and the sleeve inserts are clearly visible on the face of the barrels in the third photo. These are not original double rifle barrels, nor have they been reproved since conversion. Slug the bores and make a cerrosafe chamber cast to determine current caliber.
