Niven's father was a Scott, Moore's mother was from India, O'Toole's father was Irish and mother was Scottish.... as the bios did not list any specifics on Grant's ancestry I would assume he was the one 'more Brittish'.
I am American born, French and Lithuanian ancestry. I don't just 'assume' things before answering. If I don't know an answer, I do research on it rather than to answer in ignorance.
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David Niven was born James David Graham Niven, on 1st March 1910 at Belgrave Mansions in London - not Kirrimeur in Scotland as sometimes alleged. His father was William Edward Graham Niven and was Scottish, his mother was Henrietta Julia de Gacher and was three quarters French. David was the youngest of four children, his elder siblings in order of age were Joyce, Max and Grizel, with Grizel being the only one to outlive him. http://www.hayleydavis.btinternet.co.uk/Links/biography.html
Roger Moore was born on October 14. 1927, at the Annie McCall maternity hospital in Jeffery's Road, London, SW4.
His father, George Alfred Moore, a Bow Street police constable and Plan Drawer and his Mother, Lily Moore, born in India, the daughter of a Regimental Sergeant.
http://www.therisingeyebrow.com/biography.htm
(Roger Moore received title of SIR Roger Moore - does that make him more Brit?!)
Peter O'Toole was born on 2 August 1932, allegedly in Connemara, County Galway, Republic of Ireland but more probably in Leeds. He was the son of an Irish bookmaker father and a Scottish-born nurse mother.
http://en.wikipedia.org/wiki/Peter_O'Toole
Hugh Grant, blessed with the middle names John and Mungo, was born in London on September 9th, 1960. His father, James, was an artist who made money running a carpet firm, while his mother was a teacher (his older brother, also named James, is now a banker). Both his parents were from military backgrounds. One grandfather was in the Seaforth Highlanders.
http://www.tiscali.co.uk/entertainment/film/biographies/hugh_grant_biog.html
(He was born Hugh John Mungo Grant on September 9, 1960, in Hammersmith, London, United Kingdom. His father, James Grant, was an artist and a carpet salesman. His late mother, Finvola Grant, was a teacher of Scottish ancestry. His grandfather was in the British military service during WWII. http://imdb.com/name/nm0000424/bio)