Go to the Membership Rewards page, Frequent Traveler Participants and click on the BA Avios transfer button. Here, you don't even need to log in:
https://catalogue.membershiprewards....rtners-ba-logo

At the bottom of the page, there is a clickable bar button called "Product Information". Click on it to expand it and you'll see the information about the 30% transfer bonus to Avios.
SPECIAL OFFER From September 25, 2018 until November 25, 2018, your transfer of 1,000 Membership Rewards® points to Avios is equal to 1,300 Avios. For example: • 1,000 Membership Rewards points = 1,000 1,300 Avios • 30,000 Membership Rewards points = 30,000 39,000 Avios • 50,000 Membership Rewards points = 50,000 65,000 Avios A minimum transfer of 1,000 Membership Rewards points is required. Transfer your Membership Rewards points to Avios – the currency of the British Airways Executive Club.