Reclaiming Swiss Withholding Tax on a Lump Sum Payment under Article 18.2

I am attempting to reclaim Swiss withholding tax on a lump sum payment derived from my Swiss Pension Pillar 2 under Article 18.2 of the UK-SWISS DTA. Basel-Stadt wrote to me in translation "You will not receive a refund as the UK is excluded from this procedure." In reading the DTA currently in force (as amended in 2017), I find no regulation saying that UK residents are excluded from reclaiming their Swiss withholding tax. Earlier this year HMRC sent me the certified double taxation form needed for the tax authorities in Switzerland to complete the process. Please can HMRC offer any guidance on this matter of reclaiming Swiss withholding tax. If there is a regulation that prohibits this process, please can you direct me to that legislation.

Hi
Article 18(2) of the UK / Swiss tax treaty advises that lump sums from pensions established in Switzerland to a resident of the UK, are only taxable in Switzerland.  (1977 UK-Switzerland Double Taxation Convention as amended by the 2017 protocol – in force Updated 29 September 2021).  This means that you cannot claim the tax deducted from your lump sum from Switzerland.