Hall wasn't claiming that George was illegitimate. The King was a bastard because of the way that he behaved:
A former policeman provoked panic in Buckingham Palace in 1931 when he insisted that he had a superior claim to the throne than George V.
Anthony Hall argued that he was the 23rd descendent of Henry VIII and tried to convince mass crowds at a series of public meetings in the Birmingham Bull Ring that he was rightful heir to the throne. He started raising eyebrows in Whitehall and Buckingham Palace after making “scurrilous” attacks on the King, including a threat to shoot him.
George V, the Queen’s grandfather, was forced to intervene to make sure that Hall’s campaign came to an abrupt end, preferably in an asylum, while making it clear that the Palace’s involvement should never come to light."Forced to"? What a bizarre judgement by Jill Sherman. George was not forced to do anything. He chose to try to have Hall certified and locked up. As Sherman later writes:
On July 13 Sir Clive Wigram, George V’s private secretary, became involved, suggesting that a stop should be put to Hall’s “effusions”. He suggested to the Home Office: “Would it not be possible to keep him under observation with a view to his final detention in an Institution, without actually putting him in prison?”
Further letters showed that a summons was issued on Hall, calling upon him to find sureties or face imprisonment. Sir Clive sent a letter approving the proceedings but adding “so long as it is quite understood that His Majesty is in no way responsible for the initiation of them”.
It sounds rather like the way political dissidents used to be declared insane in the Soviet Union. The good news is that two doctors refused to certify Hall as insane.
Hall died young in 1947, but his claim was beside the point. Didn't Tony Robinson prove that Edward IV was illegitimate and find the rightful King of England somewhere in the Australian bush? It's all in Shakespeare, you know.
Richard III (Act 3, Scene 5)
Tell them, when that my mother went with child
Of that unsatiate Edward, noble York
My princely father then had wars in France
And, by just computation of the time,
Found that the issue was not his begot.