Double Taxation Treaty Relief (Form DT-Individual) is a HMRC form 'rubber stamped' by the tax authorities in the country that you live to ensure you are not doubled taxed both in UK and in the country that you live and taxed in accordance with the tax treaty.
E.g. Article 17 of UK/France tax treaty states that ALL pensions and similar remuneration is taxable in the country that you live and NOT taxable where the pension is based (i.e. Host country) with the exception of state/government backed pensions e.g. Military, civil services, state pension. This means that just because your 25% lump sum is tax free in UK does not mean it is not taxable in France. Germany's and Spains are different again.
Article 17 UK/USA tax treaty offers the same position as above for state/government backed pensions and also say's that pensions and similar remuneration is taxable in the country that you live and NOT taxable where the pension is based (host country).
That said, there is a sub-clause that says if part of the remuneration would have been tax free in the host state then it will also be tax free in the country that you are resident. E.g. 25% lump sum should be tax free in USA as it would have tax free in UK.
ALWAYS READ THE TAX TREATY OR GET ADVICE.