There are several "holy grails" in British TV history.

Lost Doctor Who episodes are one of them. Dad's Army also has lost black and white episodes (the colour ones have been repeated ad nauseam all my lifetime).

I can think of a few others. Scotch on the Rocks was a political hit piece written by Douglas Hurd showing an armed Scottish uprising along the lines of Northern Ireland. It was supposed to frighten people away from Scottish nationalism, but ended up causing copycat incidents. It vanished shortly after being broadcast probably because of its unintended effects.

The ultimate would be some of the pre-WW2 television broadcasts. Most of these were broadcast in the London area and practically nowhere else. Almost no one had recording equipment back then and they were often broadcast live.

I just wanted to mention at this opportunity that some British TV series from the late 70s, early 80s are absolutely brilliant and some of the best stuff I have ever watched. Tinker Tailor Soldier Spy, Smiley's People, Danger UXB, Sandbaggers and Sherlock Holmes are some of my favorites.