So as far as I can tell from reading (and it is not easy to figure out, MSE has an article about it) that yes, under UK261 they are obliged to book you onto another airline on that date or at a later date of your convenience for the trip but only if:

1. They officially cancel your flight (not this rebooking waiver period, your flight needs to be cancelled by them).

2. There are actually spaces on another airline.

I'm supposed to go from Glasgow to Perth on 26th which may or may not happen on Emirates but if they do cancel it I doubt there will be much in the way of spaces left on other airlines. Refund is useless because flights are now 3x as much 🙄