London & back in a day?
I look back and see you said "25 hour jaunt" but did not occur to me you meant it. Business class, no jostling for space, "running in place" ("Look at that weird passenger. Is he actually running?")*as a possibility? Are the 2 story jets (747) with spiral stairs & piano bars still around?
*some have to run away from DVTs.
Unfortunately yes- although I have been able to avoid such craziness for the last few years. Plays havoc with training. Being able to lie fully supine helps as does stretching.
On the trip to Australia flew on the new A380 which has a large upper deck- however us lowly economy class travellers were dissuaded from mixing with the upper level!