[themadturk]

British TV in that era was enormously, shall we say, frugal. As soon as they discovered videotape, which was both cheaper than film and reusable, they went all in on it. BBC especially (and ITV as well, it appears) often wiped their tapes and recorded over them to save money, since they saw no reason to think anyone would be interested in their shows in the future. For this reason, some early episodes of Doctor Who are lost forever, at least until some foreign TV station finds copies in their vaults, which has happened over the years.