Thanks Karfa, that's really helpful. Yes, I haven't cancelled the booking or taken a refund (so am effectively just asking for the difference between what I paid BA and what I paid LY).

At the time ppl were reporting a mix of 1) no re-routing being offered and 2) LH group flights. I wasn't offered anything vaguely similar to what I had booked.

I'll take a look on the main compensation thread.