Question about Roman coin

[D B Campbell]

However, the text says "CONSTANTIVS" instead of "CONSTANTINVS", does this mean the coin should be dated earlier and the man depicted is actually Constantine the Great's father Constantius?

Yes.

I am not a numismatist, but it seems that the formula "IMP C FL V CONSTANTIVS P F AVG" is rare, the V usually being expanded to VAL. The dating should be AD 305-306, but (I believe) distant mints had begun addressing him as emperor before this.

Edit: I am not sure what the gamma mint mark indicates, but it is probably not London!