Invent is a pretty concrete thing and especially in art where influence is inevitable it would be pretty clear if a new movement was started and by who.
See as how there was plenty of psychedelic rock that started exploring outside the bounds of (heavier) blues rock and simultaneously there was protopunk, so by the time Sabbath came out with Paranoid there were plenty of traces of heavy metal but there wasn't so much a band or album that defined
it like they did. they made it more concrete.
It's sort of like saying Picasso invented cubism but there were already traces of it beforehand, or like saying Thomas Edison invented the light bulb, or Columbus (or De Leon) with America.
And then that's just from what the mainstream knows. For all we know some little band in Turkey could have been tearing shit up in the early 60's but they just didn't get lucky like others did.
Uriah Heap otoh, I can see why I never got to know them more, just the name. Seems like the ABBA of metal
But even between early Sabbath and Priest they were still close to the hard rock blues riffs and production wise don't seem to sound heavier than MC5 or The Stooges.
some say Priest really started it but their early stuff sounded like southern blues. To forget semantics I'd just go with Sabbath but I know there's some unknown bands out there that were heavier beforehand, we just haven't heard them.