Happy Got-Rid-Of-Some-Troublesome-Colonies Day? :)

Needless to say, it's impossible to settle on an equivalent date for the UK as even though we're a bunch of islands, our borders have been a bit variable over the past two millennia (advance warning: what follows is the result of a merry wiki trawl!)

The Roman conquest began in 43 AD and lasted about 400 years, after which Germanic Anglo-Saxon settlers invaded and much of the region they settled became unified as the Kingdom of England (12th July 927 - the Angles called themselves Engle, hence they called their lands Engla land, which eventually morphed into the portmanteau England); while the Gaelic speakers in NW Britain united with the Picts to form the Kingdom of Scotland (~843AD). Lothian and Strathclyde were incorporated in 1124 (confirmed at the Treaty of York in 1237), Galloway was incorporated in 1234-5, the Hebridies, Isle of Man and Caithness were incorporated in 1266, then finally Orkney and Shetland were annexed in 1472.

Decoding when Wales joined the Union is tricky - the Romans administered both England and Wales as a single unit. Wales' own system of law was first codified by Hywel Dda (reigned 942-950), but following the Norman invasion of Wales in the 11th century, English law came to be practiced in the Welsh Maches. In 1283, Prince Edward conquered the rest of Wales, which was united with the English Crown by the Statute of Rhuddlan on 3rd March 1284. Wales was officially annexed by Henry VIII's Laws in Wales Acts (1535 and 1542), while the Wales and Berwick Act 1746 created a statutory definition of England as including England, Wales and Berwick-upon-Tweed.

While Geoffrey of Monmouth has the honour of the first recorded usage of Britannia major (used for the main island to distinguish it from Britannia minor, aka Britanny) in Historia Regum Brittaniae( c. 1136), the first official usage was in 1474, in the instrument drawing up a marriage proposal between Edward IV's daughter and the future James IV of Scotland.

Acts of Union 1707 which joined the Kingdom of Scotland with the Kingdom of England took effect on 1st May 1707.

Acts of Union 1800 which joined the Kingdom of Ireland with the Kingdom of Great Britain took effect on 1st January 1801, creating the United Kingdom of Great Britain and Ireland.

Home Rule Bill, which eventually lead to the formation of the Republic of Ireland came into effect on 6th December 1922, while the Republic officially became so on 18th April 1949 (and so was automatically excluded from the Commonwealth). The UK was renamed the United Kingdom of Great Britain and Northern Ireland on 12th April 1927, in accordance with the Royal and Parliamentary Titles Act 1927.