I've been giving this one more thought today and have come to the conclusion that it is unlikely this man is from the British Isles, unless he has had no family looking for him over the last two decades. If he had family in the UK who have been missing him they would surely have found his photo and info very quickly just by googling.
However, if he isn't from the British Isles then how did he get to the UK, bearing in mind he had no passport? If he came from another English speaking country such as Australia/New Zealand/US he would have needed a passport and there would have very possibly been flight records & evidence of his passport being used.
Equally, if he's European, where's his passport/ID documentation that got him into the UK? I wonder how they knew the scratched out card he had on him was a student ID but they could never identify the university/college it came from? Maybe the ID card wasn't a student ID but another kind of ID. In 1996 Germany, France, Belgium, Greece, Luxembourg, Portugal and Spain were all operating identifcation card systems. It must have have been pretty scratched out to not be able to match it to a university in the UK at the time, even if just by the colours & design.
In 1996 it would have been easy to move around the mainland of western Europe - by May 1996 France, Germany, Belgium, Luxemburg, Netherlands, Portugal, Spain and Austria had all abolished border checks as part of the implementation of the Schengen Area. He could have travelled through Europe (with his guidebook) and not needed to produce any ID or a passport except to enter the UK. Therefore if he originally came from somwhere in Europe there's no way of tracking his journey or where he originated from and his family may not have thought to check the UK.
Basically a lot of speculation, but just food for thought!