After several short lived changes between 1702 and 1707, the White Ensign of England was finally replaced by a new design showing the cross of St. George across a white background, with a Union Jack in the canton.
In 1801 it underwent a further change, consisting of the addition of the Irish red saltire to the Union Jack.
In 1864 this flag came to designate the entire Royal Navy exclusively, reserving the Red Ensign for merchant ships and the Blue Ensign for the Reserve - although from a long since British admirals tended to favor its use (like Nelson at Trafalgar).
Source: «Flags of The World» (FOTW) website, History of British Naval Ensigns Part 2(Great Britain)
Sizes: ensign W12 x L18 mm, mast flags W8 x L12 mm, jack W6 x L9 mm, pennant W4 x L48 mm.