UK Taxation Of Traditional IRA

I've seen various advice on this forum regarding the taxation of withdrawals form a traditional IRA. It's always been my understanding, from the DTA, that periodic withdrawals are taxed in the country of residence - that's the UK for me. It's also been my understanding, from the DTA, that lump sum withdrawals are taxed in the USA. Recently, I've seen advice that both types of withdrawal are taxed in the UK. Can you point me to where in the DTA, or any other document, this is defined?

Hi crowman,
There is no legislative definition of a lump sum but HMRC regards these as being any non-periodic payment of a pension.
That is, any non-regular payment that decreases the value of the remaining pension pot after such payment is made.
For example, the first (IRA) withdrawal is taken in year 1, the next withdrawal was made in year 5, and another withdrawal in year 7; such payments will not be regarded as periodic and will be treated as Lump Sum’s under the UK/USA DTA.
Whereas any amount withdrawn in set, periodic, frequent intervals (e.g. weekly, monthly, annually etc.) would not be a Lump Sum, but rather periodic payments.
Periodic, frequent, payments or withdrawals (e.g. weekly, monthly, annually etc.), then those payments would have been taxable within the UK and ‘maybe’ exempt from US tax.
This is in accordance with Article 17(1)(a) of the DTA which, again from the perspective of a UK resident, states:  
‘Pensions and other similar remuneration beneficially owned by a resident of a Contracting State [UK]shall be taxable only in that State [UK].’   
As a result, these periodic payments are fully taxable in the UK and should be declared to HMRC on the Foreign Income pages (SA106) of a Self-Assessment return.
If US tax has also been paid on those payments, then it is important to note that no UK tax relief can be claimed to offset that US tax charge against any UK tax due
 Instead, you must approach the US Internal Revenue Service (IRS) to claim US tax relief and the type of US tax relief available will differ depending on whether or not you are a US Citizen.  
Article 17(2) of the UK/USA DTA provides the US with the right to tax any Lump Sum payment which is made from a US sourced pension scheme (including IRAs).
However, the UK is also permitted to tax the same lump sum payment(s), which is in accordance with Article 1(4) of the DTA .
Thank you.