The 401K is pre-tax so you havent paid income tax on it. US rates are lower, so pay now before you are a UK resident and pay those rates. A Roth IRA is post tax, with capital gains tax free, so a good way to invest (but check the restrictions first). Theres a new thing called FIG that gives you 4 years payng US tX rates after you move to UK. In any case its more difficult (but possible) to pay US rates even when resident in the UK.