A circle is the common symbol across all bbc one idents, it links all the idents through the years. The font was usually quite formal so that it was easy to read which is important when you are looking at it over a small timeframe.
All of the idents had a globe featured in them. and there was very little fancy imagery.
As the audience grew familiar with the spinning globe being the symbol of the bbc it became a symbol of quality showing that the bbc still is the same as when it started.
In the christmas bbc idents you can see that the bbc is a family orientated channel. They started when there were only really 3 channels and 2 of them were bbc (the other was itv). They showed that they were family orientated by brightening up the usually dull idents featuring only a spinning globe. They brighten the idents by using more colour, putting snowflakes in the background sometimes they changed the spinning globe and used other images such as a bauble a present or a christmas tree.
As the number of channels increased the bbc had to try harder to appeal to its audience and to sustain its viewing figures. They have done this by using more imaginitive and eyecatching material such as ice skating penguins, judges on space hoppers and a snowball the size of a house. This has the effect of attracting niche audiences from other channels.