Following the 1707 Union Act, Scots and English Navies combined their respective ensigns to create this new one, consisting of a red flag 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 merchant fleet exclusively, reserving the White Ensign for the Royal Navy and the Blue Ensign for the Reserve - although from a long since British admirals tended to favor the use of White Ensigns (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.