I am travelling LAX-LHR-ARN next Tuesday in WTP (BA 280 and BA 782). I saw this morning that there is Avios availability in J on both legs so called up to upgrade with Avios, only to be told that the system was showing no availability. A bit more digging by the agent and her manager suggested that no availability is showing because a) my billing address is in the US (I moved to LA last year) and therefore my Executive Club account is treated as a "US account", and b) I booked through American Express US (a non-UK travel agent).

The agent suggested I could try changing my billing address and credit card to UK or possibly ringing back tomorrow in case availability had opened up to the US.

Has anyone had similar experience before? I assumed availability was availability no matter where one's EC account might be based, so long as you were upgrading an eligible booking (my LAX-LHR ticket is in T).