The common format residence permit in Switzerland is credit card-sized and has CHE and the biometric symbol in the top left corner.
Currently, the following two styles are both valid until the validity indicated on the card.
New style (white base, with a map of Europe and stars on the front): https://www.sem.admin.ch/dam/sem/fr/data/aufenthalt/infoflyer-aa19.pdf.download.pdf/infoflyer-aa19-f.pdf
Old style (pink-blue-pink with a bull and five stars on the front) : https://www.sem.admin.ch/dam/sem/en/data/aufenthalt/flyer-biometr-ausl-ausweis-e.pdf.download.pdf/flyer-biometr-ausl-ausweis-e.pdf
If your permit is being renewed or you are still waiting for it, you may inquire with your cantonal or communal authority to see if a return visa of category D can be issued (which also suffice for transiting UK).