I have tried different dates from May and the earliest is the 8.40 flight.
Probably just changed their schedule. Had the same with the ferry back from the UK. DFDS used to have one at 7.40am, but they changed it on the 8th January so it doesn’t exist now. Luckily P&O’s still does.
If I were you, I'd give Swiss a quick call because scheduled airlines (BA, Swiss, Lufthansa, etc...) change their schedules at the end of March and September. Check the flight for the date you want on Skyscanner, follow the link to Swiss from there and go through to the bookinng page. Before completing the booking, call Swiss to make sure the flight time is valid and scheduled so that you won't get a time change of over an hour closer to the date.
Regards
Ian
Just checked and there's definitely a glitch on the Swiss site because the 6am flight from LHR to ZRH isn't showing on the booking page for next week either, but it is showing on the timetable. It's also showing on the timetable page for May...
https://www.swiss.com/ch/en/Book/Fli...imeTableResult
However, the 2005 LHR to ZRH which is showing on the booking page, isn't showing for all days on the timetable. *confused*
This is the result for 11 May on which, according to the timetable, the 2005 doesn't operate.