In the UK Oi" or Hey" would usually be said in a sharp manner to bring a persons attention to something you were maybe concerned about i.e." Oi what do you think you are doing"
Hey or Hay used as a greeting would not be used in general as it would be seen as an Americanism.
"All right John" or "Hi John" etc. would only be used in a very casual friendly way." All right" is not intended or taken as an enquiry into a persons well being, just like "Howzitt" which sounds absolutely ridiculous to me.
But this is why we define American English and UK English, and as long as we can understand each other (all be it with some misunderstandings at times) what does it matter.
I would suggest from the pathetic attempt by BRUS to ridicule "All right" he should get it into his head that "Howzitt" is the variant. it's NOT English.