My one and only ride on a live seat was in a Marshall's Vampire at Shawbury in 1969. I still vividly remember being warned, as part of the seat checkout in the Safety Equipment Bay, that because it was powered by explosives and not rockets (a Mk. 3?), I would probably damage my spine if I had to eject!
Yes, the seat in the T11 was cartridge powered - from long before the days of rocket seats. I trained on the T11; the seat was MB, Mk3 according to their website. It was not as capable as some other Mk 3 seats, having a 60ft/sec gun which meant you needed 200 ft agl and 120 kt IAS in level flight. Other Mk3 seats (e.g. Hunter) were fitted with an 80ft/sec gun, which gave 90 kt ground level capability. We were told that the Vampire structure was not strong enough for the more powerful gun.
The 80ft/sec gun regularly produced minor spinal injuries, but I suspect the 60ft/sec version was a little kinder.