is it correct that if Phil the Greek had retained his Prince Philip, Prince of Greece and Denmark title at his marriage rather than giving up his Greek title and being 'just' a naturalised UK subject at his marriage he would have been King Philip?
Philip’s status as a prince before his marriage is really immaterial to the title he has as the Queen’s husband. That is up to the Queen. He is a prince in his own right because she made him a prince of the United Kingdom. That (and his dukedom) is his title.
It is highly unlikely that he would have been given the title of King in any circumstances, prince or not. Prince Albert was a prince, but not King Consort. The husband of Queen Anne was a prince of Denmark, but not King Consort.
There is some precedent in Britain for the husband of a queen regnant to be given the title of king. Philip of Spain, the husband of Mary I of England was named co-ruler when they married and called King of England. He spent most of his time in Spain, though, and never really functioned as a ruler in England. And his title did not survive Mary, who was succeeded by her sister Elizabeth.
Henry, Lord Darnley, the husband of Mary Queen of Scots, was made King Consort of Scotland.
Philip of Spain and William of Orange were co-rulers, but William's title survived his wife's death, unlike Philip's case. Mary II had a superior claim to the throne as the daughter of James II and VII (William was the son of James’s sister), and William's secondary position in the succession was recognized in the settlement which provided that if Mary died first and William remarried, any potential children with a second wife would be after Queen Mary’s sister, Anne, in the succession, thus making her, after Mary's death, the only female Heir Apparent in British history.
One of the reasons for husbands of queens regnant generally not being called King is the historical notion that King is a higher title than Queen. In many European countries where the Salic law prevailed, only men could inherit the throne and so queens in their own right were very rare. And even in countries that allowed women to reign, men took precedence in the succession until very recently (and still do in some).
It's certainly possible with more egalitarian rules about succession, the notion that King is a higher title can change and male consorts can be called King. With more queens regnant on the horizon all over Europe (Sweden, Norway, Belgium, the Netherlands, and Spain) and a move to have more equality between the sexes in these matters, we might see the King Consort title revived for the husbands of Queens.