Hi uk1.
I just managed to transfer my Airmiles Avios to BAEC.
If you go to My Account, there's a link under your balance that allows you to "Combine my Avios". You choose BA, give an account and PIN, sign your life away, and hey presto!
I got an email from BA and Avios confirming the transaction.
It looks like you can transfer any amount back and forth at will.
Mind you, they're not showing on my BAEC account yet...