I did an office transfer from Vienna to NY. It took me 6 months and a lot of efforts. Basically you need the approvals from the UK senior leadership, from the US senior leadership, you need a confirmed project, you need a exceptional track record, etc. Especially if you are trying to get into US offices like NY. If you are an Analyst I don't belive that it will work. For Consultants, Managers, etc. it should be easier given the fact that you have enough support and a network that helps you.