When is a lump sum a periodic payment ?

I am retired and a UK citizen and (now) UK resident. Having worked in the US I have funds in various 401k-like accounts. It seems that withdrawals are taxable in the US if lump sum and in the UK if periodic. Now that I have reached the 70.5 year barrier I will have to take at least the required minimum distribution yearly. Does that mean that ALL withdrawals will now be regarded as part of a variable periodic distribution? Or can I take the occasional lump over and above the minimum and pay tax at the lower US rate?

A related question: For a foreign pension, UK tax is payable only on 90%. At least for a UK pension 25%  of a withdrawal is tax free. Do I get BOTH of these reductions ie only 67.5% is taxable?

For RMDs you must use the IRS mortality and interest rate tables to calculate your annual withdrawal. You can take more that that if you want. We need to be careful about the use of the term lump sum in this context though, to the IRS a lump sum withdrawal from a retirement fund is the entire balance taken within a single tax year. So unless you are doing that you are not going to satisfy the IRS definition of a lump sum withdrawal and you will just have to pay UK tax on the withdrawals as income. As you are a UK citizen NRA there should be no US tax and no withholding.

You do not get the UK 25% tax free allowance on your 401k type funds because they are US funds and are governed by US rules, NOT UK rules. You will pay UK tax on 90% of your US pension income.