Thanks everyone for your advice so far. Whilst I always knew airline Ts&Cs were heavily skewed against the consumer, never has this been laid so bare as during the pandemic.

To clarify, these are booked on 2 separate itineraries (the flight that has been changed is for leisure, the flight afterwards is for business) So, they are not connected in any way.

.As far as I am aware however (and happy to be corrected), BA do have a duty of care to get me back home within a reasonable timeframe (few hours) of my original flight time - even if this means putting me on flights operated by an entirely different airline (even outside of the OneWorld alliance).

Within BA, I have only ever requested flights operated by different airlines that were bookable on ba.com (which the Qatar airline flights are). I have previously been moved from flights on Alitalia and KLM that were cancelled on to BA flights to get me home at a similar time to my original booking - so know that airlines do this even outside of any airline alliance.

The main unknown for me here is what happens where the travel class booked is not available on alternative airlines (which Premium Economy often isn't). Whilst I am not surprised that BA would downgrade you to Economy (and yes, it is a downgrade) rather than upgrade to Business, I am surprised they could do this without providing a refund of the fare difference between WTP & WT, as well as the taxes and fees that would have been increased based on a WTP ticket. Otherwise, I am no better off than if I had spent around £300 less and just booked in Economy!

Again, any useful advice is welcome - and will hopefully help people in this predicament in the future.