Hi matthore,
Article 18 of the UK Switzerland tax treaty:
1977 UK-Switzerland Double Taxation Convention as amended by the 2017 protocol – in force
Advises that as a resident of the UK, you are only taxable on your Swiss pensions in the UK, subject to the other clauses in the article.
If you are taxable only in the UK on your pension, you will need to request a certificate of residence:
How to apply for a certificate of residence to claim tax relief abroad
You can give the certificate to the Swiss authorities, to claim back the tax paid on the pension.
The Swiss pension would be delcared in the UK in a Self Assessment tax return.
Thank you.