Relieve pressure by cutting new outlet holes into the side of the pipe. By wrapping a collar around the pipe with an attached chamber an enclosed space is formed. Attach outlet pipe with valve to rear of chamber. Inside chamber is remotely operated cutting device. Once collar is in place and sealed then drill small hole in side of pipe to equalize pressure. Then cut hole in side of pipe then open valve. By equalising pressure before making the cut you avoid the massive turbulence from cutting hole and releasing flow into sealed chamber.
Rinse and repeat as many times as necessary to lower pressure enough to put cap on end of pipe. Then all flow is diverted out of side holes and controlled.
Advantage is pipe and chamber can be installed before high pressure flow interferes. The only part of the process that deals with pressure is the final cut.
Idea has been submitted to BP with diagrams.