401k/SS Tax

Regarding the IRA/ROTH conversion - I currently have a brokerage account and an IRA with Schwab.  They would not allow me to a) buy anything other than individual stocks and bonds in my IRA, and b) insisted on taking funds from my IRA for tax withholding/penalty (I am less than 59.5) if I did a rollover.  The latter would occur because even though I would be doing a direct rollover the funds that are withheld for taxes are treated like a distribution by Schwab.  Thus I am moving my IRA to Fidelity.  As  a side note I did start working with a company called Thun to manage my brokerage and IRA investments.  This allows me to own ETFs etc within my Schwab accounts (albeit at the cost of Thun's management fee) and have a better balanced portfolio.  We will see how Thun's portfolio performs.   

Thun were pretty non-committal regarding the tax position of a rollover conversion and whether it should be taxed in the US or in the UK.  The crux of the matter depends upon the definition of "lump sum".  It means different things to the IRS and HMRC.  I have elected to convert a relatively small % of my IRA to a ROTH, doing it every other year to avoid periodic payments (as suggested by Durhamlad and Dave B).  I think I will stick to only converting 25% in total since this correlates to what HMRC considers a lump sum.    However, I would be open to doing more if I could better understand the legalese tax language of the IRS and HMRC.

When we did our rollovers from 401k and started doing the Roth conversions my wife was with Fidelity and I was with Vanguard and we were age 55 , but living in the USA, and neither brokerage required any withholdings when doing either a rollover or a Roth conversion.  I am now managing our Roth IRAs from England and no issues in buying new funds and trading between funds within the Roths. We are both with Vanguard now, and they now use our UK mobile phone for 2FA instead of the US Skype number I used when we first moved back and gave up our US cell phones.

I have been using a dual qualified tax account in London to do my returns and we are have been doing this for the 5 tax years we have been back in the UK so feel confident in saying that lump sum conversions to Roth are only taxable in the US.  If you stick strictly to only 25% of the IRA as a lump sum then that is asymptotic and you will never ever convert all the IRA to Roth.  This year it will be 50% of my wife's IRA being converted to Roth, then the last conversion will be 100% of the remainder.

Regarding the IRA/ROTH conversion - I currently have a brokerage account and an IRA with Schwab.  They would not allow me to a) buy anything other than individual stocks and bonds in my IRA, and b) insisted on taking funds from my IRA for tax withholding/penalty (I am less than 59.5) if I did a rollover.  The latter would occur because even though I would be doing a direct rollover the funds that are withheld for taxes are treated like a distribution by Schwab.  Thus I am moving my IRA to Fidelity.

Yeah, the pre-tax contributions in the IRA you're rolling over into the ROTH IRA generate a taxable event irrespective of broker. The difference is which brokers will let us pay the withholding from other sources, which don't take any withholding, and those that want to take it from the rollover itself. The first two requires you to have cash on hand, but avoids the early withdrawal penalties.

Note that your rollover is sizable compared to your W4 withholding (which is probably zero if you're abroad) and/or any withholding applied by the broker, you may need to make an estimated tax payment to avoid an underpayment penalty when you file at the end of the year.

I'm also under 59.5, and so far Fidelity have let me take care of the taxes directly (no withholding, no age penalty), but the rollovers I've done so far have been as a US citizen on US soil. YMMV based on residency and citizenship/green card.

Thun were pretty non-committal regarding the tax position of a rollover conversion and whether it should be taxed in the US or in the UK.  The crux of the matter depends upon the definition of "lump sum".  It means different things to the IRS and HMRC.  I have elected to convert a relatively small % of my IRA to a ROTH, doing it every other year to avoid periodic payments (as suggested by Durhamlad and Dave B).  I think I will stick to only converting 25% in total since this correlates to what HMRC considers a lump sum.    However, I would be open to doing more if I could better understand the legalese tax language of the IRS and HMRC.

If they are throwing the term 'lump sum' around then wires have gotten seriously crossed somewhere. Lump sum (in both the IRS & HMRC versions) relates to distributions (as in taking money out of a tax advantaged account and into a regular account) and have nothing to do with rollovers. Ditto periodic payments. The only way I can think of that lump sum could be involved in a rollover is if you weren't doing a direct rollover (and if you're not doing a direct rollover, then again, something is seriously wrong )

Rollovers are not taxed by the UK (the UK does not tax you when you move money between pension accounts) so it's just the US that'll tax you. The main reason to do the IRA to ROTH IRA conversation over time is just to keep the US effective tax rate down. I intend to do my rollovers in December of each year, file & pay my US taxes by Jan 15th, and not have to worry about estimated payments during the year.*

* Outside of some dividend payments I need to declare from old employer stock, but I'm slowly diversifying out out of those.

Thanks DaveB and durhamlad