In general under Article 17 of the US/UK tax treaty, pensions, social security, and annuaties are subject to taxation in the country in which the individual resides. However, there are a few exceptions to this rule as follows:
(1) Any amount that is paidfrom a pension scheme established in the country in which the individual is not resident that would be tax exempt in that country will be consirdered as tax exempt in the resident country, and
(2) Any lump sum payment from a pension scheme will be taxable only in the country where the pension scheme is established, without regard to the country of residence of the individual.
So the answer to your question, your pension distributon will be exempt from UK taxation provided that contributions paid by you or on your behalf were all done prior to when you establish residency in the UK.
I hope this answers your question.
Helen