What British queens have been executed?

No reigning English Queens were executed and these are the only three consorts I know of - then again there are questions as to whether any of these three were ever legally Queens.

Anne Boleyn Queen Consort of England (1501/1507–19 May 1536) was Queen of England as the second wife of King Henry VIII. She was also Marquess of Pembroke in her own right. Henry's marriage to Anne, and her subsequent execution.
https://en.wikipedia.org/wiki/Anne_Boleyn
Catherine Howard Queen Consort of England (c. 1521 – 13 February 1542), also spelled Catherine or Katheryn, was the fifth wife of Henry VIII of England
https://en.wikipedia.org/wiki/Catherine_Howard
Lady Jane Grey (1536/1537 – 12 February 1554), also known as Queen Jane of England, was a claimant to the Kingdom of England and Kingdom of Ireland, who was de facto monarch of England for just over a week in 1553. Executed on 12 February 1554
https://en.wikipedia.org/wiki/Lady_Jane_Grey

The Kingdom of Great Britain has only existed since 1 May 1707, when the kingdoms of England and Scotland were united under Queen Anne. Previously, England and Scotland were separate sovereign states but sharing the same monarch after 24 March 1603, when James VI of Scotland became James I of England also.

The only British queens are therefore Anne, Caroline of Brandenburg-Ansbach (Queen Consort of George II), Charlotte of Mecklenburg-Strelitz (Queen Consort of George III), Caroline of Brunswick (Queen Consort of George IV), Adelaide of Saxe-Meiningen (Queen Consort of William IV), Victoria, Alexandra of Denmark (Queen Consort of Edward VII), Mary of Teck (Queen Consort of George V), Elizabeth Bowes-Lyon (Queen Consort of George VI) and the present queen, Her Majesty Queen Elizabeth II.

None have been executed.

https://en.wikipedia.org/wiki/Kingdom_of_Great_Britain
https://en.wikipedia.org/wiki/List_of_British_monarchs

Britain is the larger entity Star which consists of England, Scotland and Wales so any King or Queen of Britain would be a monarch of any of those three as well.

But the reverse does not apply. At the time of the Queens I mentioned they were Queens of England but not Scotland who had their own monarchy.

As Watch mentioned Britain didn't even exist so the Queens of England couldn't be British Queens.

"The Kingdom of Great Britain was the state resulting from the political union of the kingdoms of England and Scotland on 1 May, 1707 under Queen Anne.

It existed until 1801 when Great Britain and Ireland united. The resulting United Kingdom of Great Britain and Ireland became the modern United Kingdom of Great Britain and Northern Ireland in 1922 with the secession of the Irish Free State"

https://en.wikipedia.org/wiki/Great_Britain

But this is your question so I'm happy to answer what you want answered which means I can also add;

Mary, Queen of Scots
(8 December 1542 – 8 February 1587) was Queen of Scots from 14 December 1542 to 24 July 1567.

She was the only surviving legitimate child of King James V. She was six days old when her father died and made her Queen of Scots.

After a long period of custody in England, she was tried and executed for treason following her alleged involvement in three plots to assassinate Elizabeth I of England and place herself on the English throne.

https://en.wikipedia.org/wiki/Mary_queen_of_scots

Although Great Britain has only existed since 1707 through the Act of Union between England and Scotland the islands have been known as the Britain for much longer. Althouh latterly restricted to the main island it originally refferred to all the islands in the group including Ireland and the Isle of Man so there are probably a lot more queens to be included in the list. Interestingly Wales is usually only referred to as a principality, whereas as the England, Scotland, Ireland, the Isle of Man and even Cornwall and Fife in Scotland have or are referred to as kingdoms. There is still an ongoing debate as to whether Cornwall is actually part of England.

https://en.wikipedia.org/wiki/Constitutional_Status_of_Cornwall

https://en.wikipedia.org/wiki/Britain_(name)

The British Isles have been called Britain or Britania or some such since antiquity.

https://en.wikipedia.org/wiki/Roman_Britain

So you would count, for example, Boudica, a queen of the Iceni tribe, as a British queen?

https://en.wikipedia.org/wiki/Boudicca

So we would have to go back into antiquity looking at every single tribe who ever occupied the British Isles and determine which had queens and which of them were executed? Include me out!

Ancient Britain (and Britons)

I, Nennius, disciple of St. Elbotus, have endeavoured to write some extracts which the dulness of the British nation had cast away, because teachers had no knowledge, nor gave any information in their books about this island of Britain. But I have got together all that I could find as well from the annals of the Romans as from the chronicles of the sacred fathers, Hieronymus, Eusebius, Isidorus, Prosper, and from the annals of the Scots and Saxons, and from our ancient traditions.

http://www.fordham.edu/halsall/basis/nennius-full.html

Brit·on ?bri-t?n
Function: noun
Etymology: Middle English Breton, from Middle French & Latin; Middle French, from Latin Britton-, Britto, of Celtic origin; akin to Welsh Brython
Date:13th century

1: a member of one of the peoples inhabiting Britain prior to the Anglo-Saxon invasions
2: a native or subject of Great Britain ; especially : ENGLISHMAN

mw1.merriam-webster.com/dictionary/Briton webpage no longer exists

Unless Star modifies his question with a limiting date, I would go back as far as possible. This is how we now study "Native American" cultures that existed before Amerigo Vespucci was born. Oh, actually, yes and no, Watch -- Boudica was a British queen, but as far as we know, she wasn't executed. She committed suicide before that could happen.

The splitting of hairs is getting very fine here. While technically correct re Britons and the old name of Britain etc the true essence of being British and the use of the word to describe someone as British came from being a part of Great Britain when it was formed.

Thus a British Queen is one who was Queen of Great Britain. Just try telling a Scotsperson that Katherine of Arrogan was their Queen or the English that Mary I, Queen of Scots was theirs. They were two distinct countries with their own Royal Houses

"Although early assertions of being British date from the Late Middle Ages, a developed British national identity emerged following the creation of the Kingdom of Great Britain in 1707.

The notion of Britishness—which was closely tied with Protestantism at the time of its inception—was forged during the Napoleonic Wars between Britain and the First French Empire, and was developed further during the Victorian era.

The complex history of the formation of the United Kingdom created a "particular sense of nationhood and belonging" in Britain; Britishness came to be "superimposed on to much older identities", and the English, Scottish and Welsh "remain in many ways distinct peoples in cultural terms", giving rise to resistance to British identity."

https://en.wikipedia.org/wiki/British_people#Sensitivity_around_use_of_term