There are other aspects to the EU apart from the Euro. Freedom of movement, freedom of trade are two of the biggies. No pressure was put on us in the UK to join as far as I know, thank God.
The eastern countries are/were in the process of complying with the requirements for joining the Eurozone monetary union.
As for Britain and Denmark… Well, the City of London already controlled their financial affairs. And they knew that the Euro was going to destroy sovereignty in Europe by destroying itself… It was the idea. So they were granted an exemption from the Euro.
Northern Europe is controlled from the City of London… Continental Europe is controlled from Rome, i.e. NATO and the EU.
The City contains the Crown Corporation, the Bank of England, the London Stock Exchange and Lloyds. i.e. All the wealth of Britain, managed of behalf of the monarch.
The City has effective self-government from England and the UK… A corporate charter almost a thousand years old, which it has the power to redraw if it wants to. In many ways, it IS the British monarch - her real interests, anyway. No “elected” British government has the power to harm the monarch’s interests, obviously.