Christopher Reeve returned to Britain for the first time since the 1995 horse-riding accident that left him paralyzed. “I always said that on my 50th birthday I would stand up and thank everybody who had helped me,” Reeve, 47, told reporters Saturday during a visit to London’s Millennium Dome. (The actor took up residence in Britain for 10 years after he made “Superman” outside of London in 1977.) He toured the Dome with his wife, Dana, and two children, Alexandra, 16, and Will, 7. Also on his calendar was a reunion with about 50 old pals and an appointment with a London-based researcher, Martin Schwartz, who soon expects to start trials of a new treatment aimed at restoring mobility to spinal injury victims.