I am a first year consultant within a Big Four firm in London, however I am considering a move to the States and have vague connections at Accenture and Deloitte in addition to my own firm. Would I be most likely be made to start from scratch at either firm or could I enter as a second year? And how is ACN Strategy regarded in the UK and the US? Do they rival Deloitte Strat or even PwC/EY Advisory?

You'd start as a new graduate entrant. Bear in mind that you would need to get a work permit independently as firms will not sponsor you (there is a plentiful supply of unemployed graduates in the US). ACN and Deloitte are broadly on par. PwC is on par in the UK, less well-known for consulting in the US. EY is less-established or reputed for consulting in the UK, and is building from a low base in the US.