If it's possible to cancel Article 50, then that's viable and can be sold to the public. But joining the Euro is economically insane
I don't think that joining the Euro would be forced on the UK should some arrangement to stay in the EU be drafted. Apart from the politics of selling the deal to the Brits (everybody would scream right up until they found the coins still had a picture of the queen on them), there are politics of the Eurozone and the future direction of the currency to deal with. But I also don't think it would be economically insane either. UK fiscal policy is closer to that of Germany than that of Italy and it hasn't favoured devaluation for years because it has a labour market that is flexible enough to deal with competitive pressures directly.
But let's not put the cart before the horse.