Yes it's a bit difficult to make this one out. Normally BA flies there and back to the USA. Now if your flight had been caught in a previous day's turmoil and was therefore not available to fly LHR to California and back again, then that usually is OK for EC261. If there were other services from that mystery airport, especially Virgin, flying then that's a good clue that EC261 may be available.

But you can just apply as per the posts at the start, and then react of any reply.