From my understanding it doesn't matter if your WT+ fare is flexible - they are all upgradeable with a £100 change penalty (always included in the upgrade price quote)...

That said, I have been offered an upgrade from WT+ to CW on this route (LAX-LHR) for ±£4,500 one-way... On that flight, a J one-way ticket came in at roughly £1,000 less. It seems to happen disturbingly regularly that they 'misquote' the upgrade offers...