There wont be some announcement of a federal state, I suspect it will begin with the implementation of light weight tax harmonisation programmes throughout the 'Eurozone' countries gradually moving to a more consolidated position in around 5 years (the use of an eu tax identification number is an example of the foundations being placed) - as I understand it the extent of the UK's veto on Eurozone matters is minimalistic.
They're won't be any obligation to join the euro for the uk but ultimately the reparatory work on the political and fiscal framework that is needed to support the euro will be vast and there will be an overspill in many areas on fiscal elements (before political) that will impact the non Eurozone members - if the eu is headed for a two tier system then the weighting of preference on the framework needed with the ECB, the regulators, the bond market and the banking sector will always go to the Eurozone. The amount of political capital that has gone into this currency is unprecedented we have all seen what they have been prepared to do to a sovereign nation state in Greece, Portugal, Spain, Italy, Ireland and Cyprus over the last 6 years to ensure that nothing deviates away from that.