Hi all,
I've just managed to complete this on iOS/iPhone without any issues.
I've put some instructions/screenshots below for anyone struggling, hope this is helpful.
1. Tap on Account and then Settings
2. Tap on the option for BAEC
3. Tap on Link Account
4. Sign in using your BAEC details
5. If you are happy with what is being transferred from BAEC to Uber, then click Accept
6. Once done, you should see the following screen