Thanks for the hints about Northern California.
I see that OSL-LHR-SFO and back in Y would get me 24,000 Avios and a few TPs. For us, that's more than 3x RFS tickets to ZRH and back in ET.
Three return revenue ET tickets to ZRH would cost from £390, compared with just £330 for the SFO trip. This is almost getting to the stage where it would be impolite not to book SFO.
Yes, I need to add LHR-OSL-LHR, but leave me contemplating. It's just that a long journey in Y didn't figure in my plans until yesterday. But I haven't flown in a 787 yet ...