If you have an American Express Membership Rewards account, you can transfer points to participating frequent travel programs, one program being your British Airways Executive Club account. Usually one Membership Rewards point equals one British Airways avios. So 1,000 American Express Membership Rewards points will yield 1,000 avios.
However, now through 9/17/2017, you can receive 40% more Avios when you transfer Membership Rewards points to your British Airways Executive Club account. So during this offer, 1,000 American Express Membership Rewards points will yield 1,400 avios.
If you are trying to accumulate avios points right now, this may be something you may want to consider taking advantage of.