British Airways will allow you to spend Avios with partner carriers such as Alaska Airlines and American Airlines. The nuts-and-bolts of converting points is, naturally, boring. But for the purposes of this hack all you need to know is: rewards are distance-based, and 12,500 Avios convert to a free economy seat on any flight between 2,001 and 3,000 miles long.
So, at 12,500 Avios per leg, you can get four seats of 2,001-3,000 miles. Or two round-trip tickets from the Mainland to Hawaii.
On Alaska Airlines, you can fly from Anchorage, Alaska; Bellingham, Washington; Oakland; Portland, Oregon; Sacramento, California; San Diego; San Jose, California; or Seattle.
On American Airlines, you can leave from Los Angeles or Phoenix. (American also operates flights from Dallas, albeit for 20,000 Avios a pop.)