I believe Article 18 of the bilateral Swiss-UK double taxation treaty covers this:
"1. Subject to the provisions of paragraph 2 of Article 19, pensions and other similar remuneration paid to an individual who is a resident of a Contracting State, shall be taxable only in that State.
2. Notwithstanding the provisions of paragraph 1, a lump sum payment derived from a pension scheme established in a Contracting State and beneficially owned by a resident of the other Contracting State shall be taxable only in the first-mentioned State. "
Not sure if this would be updated/amended with all the other tax accords the Swiss are now in the process of updating due to the recent pressure on the Swiss re. UBS and others.