For some reason, this reply specifically cemented the argument for me. Thank you - I now agree with you.

Edit: If it helps, my confusion was the appearance of causation from I-refuse-the-£10 to I-receive-the-£1e6. When you made this comment, I mentally went back and saw that the fraction of possible worlds in which Alpha gives me the million is unchanged by Omega's prediction, and therefore that I can take the tenner without affecting it.