I'm flying LHR - HEL - BKK.
I've managed to upgrade to business on the HEL-BKK leg in advance (so fare bucket U). However LHR-HEL is still economy. (If it matters, my entire return leg from BKK to LHR is all in business, but on a different day.)

Does anybody know if I'll be able to use the lounge (CX / one of the oneworld lounges) at LHR? And will I be able to use fast track security, other business perks at LHR? I have no status (other than BA blue).

I did read somewhere that the long haul segment is the one that matters - but not sure if this was accurate/up to date.
Thanks