Denmark, Latvia, and Lithuania are sort of partway in the Euro: they joined ERM II, which guarantees a fixed exchange rate to the Euro. So while they retain separate currencies, those currencies are just alternate units for the Euro, at a fixed conversion, unless they break the peg, and how they rise/fall against other currencies like the dollar or yen is 100% determined by how the Euro moves. Though admittedly it'd be easier to break the peg than to unwind full Euro membership, so the exit strategy is clearer.

I believe Sweden and the UK are the only two non-recent EU members with fully independent, floating currencies.

I forgot about all the new eastern and central European member states that are in EU but not eurozone, eg Poland, Romania, etc. Most of those joined the EU after the euro started, and so couldn't join eurozone, though many are starting the process.

So will the UK be well protected from this fallout?