and the only thing they could come up with was that the system flagged me because I was travelling to England with no onward ticket This is quite likely the is

and the only thing they could come up with was that the system flagged me because I was travelling to England with no onward ticket This is quite likely the issue. The UK are an extreme stickler in this regard. When you fly from Zurich, you need to show your passport to leave the Schengen part of the airport and in addition there are strict controls at the gates for UK bound flights, where your id is checked again. Contrast this to flights within the Schengen room, where you scan your boarding pass (mobile, home printed, or airline issued) at a couple of automatic gates and you're in the plane. No id at all required. From what I've read it's more and more the case that countries use airlines as cops to deny entry at the point of origin. If somebody with invalid travel documents, or missing visa gets onto the flight the airline can be heavily fined.