According to its web site, George Marshall is a visiting lecturer at the University of Glasgow, is not (as far as we are aware) a creationist activist in the core creationist movement in the UK. He was not one of the signatories to the 2002 Estelle Morris letter and appears to have no active involvement in any of the UK creationist organisations.
According to the Times Higher Education Supplement, he is a lecturer in neurobiomedicine who claims the complexity of the eye makes him "balk at evolutionary theory". The complexity of the eye has been thoroughly demonstrated to be consistent with the theory of evolution, btw.
His creationist testimony is at http://www.answersingenesis.org/creation/v18/i4/eye.asp, he is believed to be Pentecostal. University of Glasgow lists him only as a visiting academic: http://www.gla.ac.uk:443/ibls/staff/honorary_nbs.php. It is unlikely that he has retired of old age as he is believed to be in his 40s. See also his staff web page at http://www.gla.ac.uk:443/ibls/staff/staff.php?who=TeePd|~
According to his bio on the Answers in Genesis web site, Marshall is now Sir Jules Thorn Lecturer in Opthalmic Science. He obtained his first degree in biology from the University of Sheffield, after what is understood to be a very serious illness. He then obtained a Master's degree from Sheffield and, eventually, his PhD in Opthalmic Science from Glasgow in 1991.
Marshall is unusual amongst creationist scientists in that he actually understands biology.