If you are retired and have planned your income sources it's possible to generate a lot of tax free income. The unfortunate thing if you are a US citizen resident in the UK is that you only get to use the lower tax free allowances or tax bands......and those tend to be in the US.
In the UK the first £10k of income is tax free, there is no tax on the first £11k of capital gains and the effective tax rate on dividends is 0% if you are a basic rate tax payer. You can also take a 25% tax free lump sum from a pension plan and the ISA offers tax free growth and income.
Being a US citizen gains in an ISA are taxed and the tax free nature of the 25% lump sum is debated, but as you can now take it as a series of payments over a number of years it looks less and less like a lump sum (which is never defined in the treaty anyway) and more like tax free periodic payments. In the US the personal exemption and standard deduction makes the first $10400 tax free, although circumstances and itemized deductions could push that up and you pay no tax on capital gains or dividends if you stay within the 15% tax bracket. You also have the ROTH which is tax free in the US and the UK.
So if you can just take $10400 in income and then get the rest of your money from capital gains, dividends and ROTH withdrawals you won't pay any tax in the US or the UK. That might not sound like much income, and if you start SS or UK state pension 0% taxable income threshold will probably be breached, but you can live well on not much money if you've managed to pay off things like mortgages and other debt. The other sink for money is a car, so if you stop commuting that saves a ton of money too.
Thanks to the great info from the good folks on this site I've been able to do some advance planning ahead of our move. (UK tax year 16 - 17)
I have private pensions which take me well in the 20% UK bracket, but my wife does not have any pensions. We have now moved all our after-tax investments into her name (no cost just some paperwork) and by the time we move they will all be HMRC Reporting stock index ETF's. All the bond funds are in our IRAs and ROTHs. We have a substantial sum in I-Bonds which are tax-deferred and they are mostly in her name (the accumulated interest is taxable when the bonds are cashed in).
After the move all the extra money we need on top of my pensions will be drawn from her accounts to maximize her tax free allowance, and favorable taxation of stock dividends and cap gains.
I will make another ROTH conversion this year and next from my IRA, and once we are in the UK we can make some ROTH conversions of my wife's IRA up to the top of the UK 20% band.
.... and I thought I'd need Sudoko's and crosswords to keep the brain ticking over in my retirement.