Hi everyone, just thought I'd give a little update. I called Tesco who said they could provide cover and then after I called them up a second time to check something then said they wouldn't be able to.
Churchill and the Post Office both said yes although the PO sounded like they were very much reading from the policy document and didn't really have a clue otherwise. When I mentioned to both about other insurers saying they couldn't cover due to known risk, they said as long as your flight hasn't been cancelled and then you're trying to get insurance, that's fine.
In the end, I went for Coverforyou - woman on the phone sounded super knowledgeable about why I'd want that kind of cover (whereas the others sounded as if it was the first time they'd been asked about it which was odd). Make sure to take out their extra travel disruption add on. Was told you're covered for lost accommodation costs if your flight is delayed over 12 hours.
Also in my deep dive into what you can claim back in this situation, the airline apparently is required to get you to your destination that same day even if it's by getting you on a flight by a different airline. And if you book your flight on your credit card, you can try to claim it back from your bank if anything happens.
Also if your rescheduled flight is over 2 hours later from your original one, you can claim back compensation. £220 for short haul flights per person: www.caa.co.uk/passengers/resolving-travel-problems/delays-and-cancellations/cancellations/
And if your flight back is cancelled, you should be able to claim back reasonable accommodation costs back from the airline (make sure to get receipts as they don't accept card statements!).
Now I definitely need a holiday to get over the stress of trying to find travel insurance! Safe travels to you all!