That's correct - you need a PPL to commence either an EASA or a UK CPL, but it doesn't matter where you got that PPL so long as it's ICAO compliant.
Also correct that you can't automatically fly PiC in a G-reg aircraft without a UK licence, but it is possible to get a temporary approval to subject to an examiner sign-off. Here are the procedures:
https://www.caa.co.uk/commercial-ind...al-activities/
You technically don't need anything at-all to start an integrated fATPL course but my advice to anybody would be to do a PPL first, to understand if you have the dedication and aptitude, before you commit to anything more.
If you have the right to live and work in both the UK and Switzerland, as your post suggest, I'd suggest looking forwards to dual rated schools that will enable you to get both a UK and an EASA
professional licence at the same time. No need to do that at PPL level.
G