A friend of mine found these keys in the garden of a house in London UK, would anyone be able to provide any information on what kind of locks they fitted or history about them, including perhaps if they have unique serial numbers that could be traceable back to the original owner ?
Keys almost never have "unique serial numbers that could be traceable back to the original owner". (Think about it: If someone found your keys, isn't there a hazard they'd "return" the keys at 3AM sometime and help themselves to a reward? This is why "please return" tags should always go through a trusted third party.)
Interesting that these are all keys from American lock companies. Either someone had a bunch of imported locks, or whoever lost the keys in the UK was a tourist. I would presume this means you've got close to zero chance of finding the owner.
Some of the numbers you're seeing are "open codes" -- a direct indication of what the cuts on the key were when it left the factory. (Both the Corbins.) Some appear to be manufacturer's codes, which provide the same kind of information in less obvious form.
Yes, the "C" is a G. The "Cent sign" is a stylized ampersand. The "ID" on the Corbins appears to be 1D.
I'm not entirely sure what you were expecting. The three longer ones are for pin tumbler locks almost certainly mounted in a door. The small one is likely for a padlock. From there appearance I would guesstimate that they are somewhere between 10 and 30 years old.