It is still possible to book SIR-LCY-SIR at the Powdair website and the flights are shown under the airline code F7. However, the slot request at LCY for those flights was withdrawn. I made a trial booking (of course without the payment!) and got a quote for the return at GBP 443.62; not a bargain.
One other point I never considered but going thru the booking process, I realized that the site is available in English only. This shows either stupidity or arrogance. Just expecting that every single customer in the Valais speaks English is not a good sign of understanding the market and the people in the catchment area. And focussing on incoming only would be dumb too!