Advice needed please.

My TLV-LHR-GLA flights were cancelled. Chatline person said there was nothing until 13 May - I asked twice for him to check El Al availability to London but he swore there was nothing. (I have the chat transcript.) He rebooked me for 15 May and said any further changes would be subject to fare rules.

Then I went to the El Al site and found a flight to Luton on 12 May. I booked it.

So now I am out of pocket for that, plus BA won't refund my return fare, which was mostly Avios, but I'd still like them back if I can't get BA to pay for the flight they should have rebooked me on. What's my best course of action here? How can I avoid an AI-fuelled auto-reject to my compensation request?

I do have travel insurance but they insist I do whatever I can to reclaim from the airline first, which is fair enough.

TIA.