People in the UK voted to join the EC, not the EU. British leadership (Margaret Tatcher) was very straightforward from the begining about not joining the EMU/Eurozone.
But they were in the EU, it's not about the intention of not being in. Also EC is a predecessor of the EU. It also started with the BeNeLux before that
Exceptions on joining, but not participating 100% shouldn't be allowed in the first place
Remember, people voted to join the EC, and politicians then signed the UK up to the EU without asking the people. The UK was never meant to be part of the EU. The UK & US helped start the EU to prevent Europe from falling back into war, but it was never meant to engulf the UK, who as Churchill said, is too incompatible to ever be a part of it.
Exceptions on joining, but not participating 100% shouldn't be allowed in the first place