If and when Prince Charles becomes king, what will be his official title?, as in King “who the what”. At one time I read an article that said he would not become King “Charles”, but something else.
He’s entitled to choose whatever regnal style he likes - he could be “John II” if he wanted, although I think that’s unlikely. 
The article you’re thinking of was published in the Times last year, suggesting he might take the throne as “George VII”, in honour of his father.
If he does so, he’ll break an interesting pattern, going back to 1830:
William IV
Victoria (I)
Edward VII
George V
Edward VIII
George VI
Elizabeth II
Charles III and Henry IX would give us the “straight” of regnal numbers from 1 to 9. Would mean William having to abdicate before his accession, of course - not sure if that’s theoretically possible.
Grandfather, rather. My apologies.
So Elizabeth III isn’t even being discussed?
George VII, by the Grace of God, of the United Kingdom of Great Britain and Northern Ireland and of His other Realms and Territories King, Head of the Commonwealth, Defender of the Faith*
*Charles has expressed a desire to be “Defender of Faith” instead of “Defender of the Faith”, but this would take an act of parliament to change.
IIRC from my European History class, Defender of the Faith was originally a title granted upon Henry VIII by the Pope for Henry’s support of the RCC during the early years of the Reformation. Which Henry then co-opted to refer to Anglicanism when he decided to become a schismatic himself.
–Cliffy