Feel mildly jealous of all these ex-British colonies getting an independence day. We get nothing of the sort in the UK!
Well, you could be like Australia and celebrate the day on which a foreign invading force landed and decreed that “This is all ours now”. William the Conqueror land on 28 September 1066. More recently, William of Orange landed on 5 November 1688, but (a) 5 November is already remembered in the UK for other reasons, and (b) November’s not such a great time for a public holiday, weather-wise. So maybe go for 28 September.
Weather is a little borderline that late in September.
What about repelling an invasion? We could go for Battle of Britain (start of, a barmy 10 July) or the Spanish Armada (also July).
Trying to be funny, not trying to be mean …
Soon enough the Scots may be celebrating their own Independence Day. Which the rest of the UK could celebrate as “Good Riddance! Day” Shortly thereafter the Welsh may follow suit and now the UK can celebrate two Good Riddance days per year.
Which of course will get the Northern Irish all uppity and soon enough the UK will no longer be united, and England can celebrate 3 Good Riddance Days. 
The usual Battle of Britain Day is Sept 15, when That Man is supposed to have decided to postpone the invasion indefinitely. It has been suggested as a possible public holiday since we are rather short of one at that time of year.
As for other countries, what about all the successor states to the Hapsburg, Russian and Ottoman empires? Finland, Estonia, Latvia, Lithuania, Poland, Czech Republic, Slovakia, Bulgaria, Romania, Greece, for starters. A quick Google suggests some of them have a variety of political commemoration days, and of course, Austria and Hungary are in an ambiguous position. Then there are the former Yugoslav states, and all the Arab states.
You have Guy Fawkes Day, a holiday named after a criminal. What more do you want?
We kicked the Brittish out of Buenos Aires twice (1806 and 1807), but we don’t have a national holiday for neither occasion, which is downright un-Argentinian if you ask me, I think I’ll start working on a proposal. (We could celebrate it on June 22…)
There’s always the day the Argentinians kicked the British out of the Falklands, appropriately April 1st (1982).
It was April 2 and it’s a holiday (Veteran’s day).
However that didn’t stick like the 19th century ones.
Federation day is a holiday in Australia. They chose to celebrate federation on a holiday precisely because they thought it was an occasion for celebration.
Sure, but federation day did not mark the end of British involvement with Australia, any more than Canada Day (July 1) does in Canada. Both of them are to commemorate the formation of the countries, not the end of British legal authority. The Australia Acts, which ended British authority, came into force on March 3, 1986.
Scotland and Wales already have national days (as does Ireland, which doesn’t celebrate the British leaving any more than it celebrates them arriving). But I like the idea of good riddance days; perhaps the UK could celebrate a good riddance day for each country that used to be a British possession but decided not to be any more? It would make for a lot of public holidays in the UK, which would be an unqualified good thing.
Yes. That’d be fun. Ref my comments farther upthread I think I’ll start checking that website to find a holiday to celebrate every day of the year. Even if I never do figure out exactly what they’re commemorating or how they typically celebrate.