The union flag was devised to enable private companies to raise "national" armies under an English flag in order to enslave and exploit sovereign countries they had illegally invaded and colonised. The Union Jack was initially a maritime device.
Prior to the introduction of the Union Jack (a jack being a naval flag flown from the "jack-staff" on the bow of a ship), pirate ships (or even convoys or armadas) with English Royal charters often flew their own colours and ended up engaging each other in battle despite being on the same "side". In reality, the only side they were on was their own - they were purely commercial ventures and having the Royal charter just meant that for a percentage of the take, the Crown did not prosecute them for theft, piracy, murder on the high-seas, slavery, etc.
The skull and cross-bones device, commonly touted in films and TV as the "pirate flag" is a propagandist attempt to discredit the Templars as lawless brigands around the time Philip got the nod from the then Pope to end their power and thus Philip's and the Church's huge debts to them. The original pirate flag is in all probability the Union Jack and not the so-called "Jolly Roger". (discussions about whether the skull and cross-bones is Templar or Masonic or whether Masonic or Templar are different are separate

)
There is evidence that the Union Flag, having been used successfully at sea to identify ships with a common purpose, was introduced on land in order to prevent situations where for example Cecil Rhodes's private army flying the British South Africa Company flag (remember BSA bicycles, motor-bikes, rifles, air-guns,?) attacked other "British" commercial contingents in Africa because they didn't recognise each other's flags / emblems. This cost the Crown and the commercial operators money so the privateers sought and were granted a Royal charter to allow them to use the English pirate flag on land, thereby improving cash-flow.
The source of the design of the Union Flag is another huge propoganda lie. It has nothing whatsoever to do with the crosses of various national Christian saints being amalgamated and built into a "Union" flag. The English cousin of a Russian Tsar saw the Russian "Jack" on a fleet of Russian ships, decided it was pretty and copied the design, changing some of the colours slightly in doing so.
http://en.wikipedia.org/wiki/File:Naval_Jack_of_Russia.svg
This helped Russian and English companies with Royal charters to recognise each other's ships when they were transporting opium and slaves and avoid attacking each other's ships (such attacks upset their respective Emperors, shareholders and underwriters at Lloyds.)
At the start of the American Civil War, it was copied again, becoming the jack of the Confederate Navy.
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The reasons for the similarities should be obvious as the English wanted their lucrative slave-trade to continue and were not enamoured of Lincoln's plans to end slavery and slave-trading.
I see the Union jack as a device that is intrinsically English, that has unified world-wide commercial interests, giving monetary matters precedence over human rights and the sovereignty of weaker nations, using nationalism as a subterfuge and as a sop to the masses.