Let's start with the easiest part first, the cartridge headstamp.
K = Kynoch (Ammunition Factory Located in Birmingham, England)
51 = Year of Manufacture
7 = Mark 7 (ie, the Brits had "Marks" of their cartridges too!)
The Mk 7 round uses cordite as its fuel, a 174 grain Spire Point flat bottomed bullet, and unless I had positive proof to the contrary, I would assume a corrosive primer, even as late as 1951.
"On the barrel its says: NP 303 2-22 18.5 tons"
NP means it was proofed with "nitro" (ie, smokeless) powder
The rest of that line is a description of the .303 cartridge
.303 = nominal diameter in inches
2.22 (also seen: 2.222) length of the case in inches
18.5 tons = proof pressure
"8 over 1942" sure sounds like the month / year of manufacture.
I believe the "MO120" (on the butt socket and repeated on the magazine and stock) is the serial number, but it seems too short to be a s/n. I'll have to dig through the books...