Hi,
Just saw this and thought it was worth letting you know a recent comment from HMRC on their Community Forum. I posted the full text on lump sums elsewhere, but thought this paragraph answers your question:
A UK resident, Article 1(4) above permits the UK to tax any US sourced lump sum payment received, as if Article 17(2) of the DTA was not in force or applicable, Article 1(4) effectively ‘overrides’ the provision at Article 17(2), and the consequence is that both the UK and USA can tax any lump sum payment received from a US sourced pension scheme.
I have challenged the HNRC comment (is hit law or is it an agents opinion?) but still waiting for a reply.
G