Tirana1 to answer your question “I don’t get why a 777 or mid haul A321 could not have been deployed” It’s all to do with flights crews, and the hours they can work.
There was most probably a crew in Almaty ready to fly the outbound B767 (regardless of configuration) back to LHR, as the outbound crew would be out of hours on arrival. If a A321/B777 had been deployed, whilst it would get you there, the inbound service would need to be delayed until the crew had sufficient rest to fly back again. Commercially not really viable.
I am sorry to hear that you ‘experienced’ the B767, I had a similar one a couple of years ago to Montreal, where the B767 I was sitting on was declared non-serviceable due to a previous lightening strike, so we spend 3 quality hours in T5B and then flew in a Euro-configured plane to YUL. It didn’t matter that much, as it was a day/evening flight. More of an issue was there were no amenity bags loaded, so Mrs pheighdough didn’t get her Elemis bits and pieces!!