In the following sentence, would you use "English" or "Great Britain" in both places?

Just prior to the War of 1812 between Great Britain and the U.S., Spain allied herself with British interests.

It should be Great Britain, which was formed by the union with Scotland in the early 1700's.

I would say that your terminology 'Great Britain' and 'British' is correct as you have phrased it there.

After the Act of Union in 1707, England and Scotland were united in the Kingdom of Great Britain. After that date it is not correct to refer to England (or Scotland) as a separate entity in foreign affairs.

Strictly speaking, I suppose, it should be "the United Kingdom (of Great Britain and Ireland)", which was officially formed in 1801, but I don't think there's any real objection to "Great Britain".

Very often Britain is used for United Kingdom of Great Britain and Northern Ireland. So I'd opt for Britain and British.

Edited to add: To foreigners it must seem that we suffer from some excruciating identity crisis in these islands.

In the late 1980s I read an amusing article on country names. Among other things it suggested that unduly long country names may hide a problem.

Shouldn't the first option be "England" and not "English"?

England refers only to the that part of the United Kingdom that is left if you 'take away' Wales, Scotland and Northern Ireland. (This is a negative defintion but is easily understood).

I remember hearing about a foreign visitor to Britain (in the 1950s) who began his talk to a conference in Edinburgh by saying, 'It is a great pleasure to be in England for the first time'. His audience was far from pleased - and showed it, too.

Would you ever say "Great British" as an adjective?

In some formal international contexts the adjective is Britannic. Ages ago, when spending a year abroad I received a letter from a government department in Britain, and the envelope had no postage stamp but was sent On Her Britannic Majesty's Service. (Yes, I know it sound oh-so-important, but it was about some minor technical matter).

It says Britannic in our passports too. "Her Britannic Majesty's Secretary of State Requests and requires in the name of Her Majesty all those whom it may concern to allow the bearer to pass freely without let or hindrance, and to afford the bearer such assistance and protection as may be necessary."

When I was in Japan I stayed with a Samurai family that had a coat of arms so I showed them my passport with the Royal Coat of Arms on the front and that statement was translated to them as meaning that if I was impeded or harmed in any way, The Queen would send a gunboat to help me - they were most impressed as the Emperor of Japan says nothing like that about them, lol.

Thanks. Until 1949 or so passports used to have the name of the Foreign Secretary and a facsimile signature. I must say I smile just a little at 'We, Ernest Bevin, ... request and require ...' in an old passport issued to my father.

Until the 1870s or even later, most passports were actually signed by the Foreign Secretary in person.

I just looked at mine, the burgundy flimsy thing - I much preferred the smart solid navy one - this is headed European Union (which we are not in!), then British Islands, then Bailiwick of Jersey.

Inside the cover it says Her Britannic Majesty's Lieutenant Governor and Commander in Chief of Jersey (Channel Islands) and its dependencies Requests and requires... then the same as Flopsy's.

Since we are not in the EU, despite the front of the passport, those islanders who do not have a least one grandparent who was born in the UK have a stamp in their passport telling them they do not have the right to work or live in the EU, other than in the British Isles.