Oxygen needs
Air is a mixture of components the proportions of which change only slightly with altitude. For humans the two most significant in the short term are oxygen and carbon dioxide, oxygen makes the systems work carbon dioxide has control influences
The proportions may considered as having partial pressures in accordance with their fractional volumes. Normal air mix provides sufficient oxygen partial pressure up to about 10,000 ft although some reduction in body functions are detectable above about 8,000 ft. Above 10,000 ft increasing percentages of oxygen are required to be the equivalent partial pressure of air at 10,000 ft. Above 25,000 ft the equivalent partial pressure of even pure oxygen is insufficent because the overall pressure is so low that is less than the partial pressure would be at 10,000ft
The answer is to breath pure oxygen at increased pressure. This is called pressure breathing and is available to military crews who fly at these altitudes for short periods should the cabin pressure fail or the pressure hull be penetrated by enemy fire. In combat it is common to reduce the pressure setting to a lower level to reduce explosive damage from hull damage and the severity of cabin altitude change should it happen.
There are systems in place to allow crews to use high pressure oxygen but they involve carefull individual fitting of specialised equipment and are not practical for commercial flights. Pressure breathing is very tiring and ordinary oxygen masks don't work because the pressure makes the throat swell. The whole head must be encased.
There were other RAF aircraft using these systems but sadly they are gone. Those were the days