and not to confuse the public if the train was going on the bottom half on the circle the train describer would show a Putney Bridge destination and if it was travelling on the top half of the circle then they would show either a Baker Street or a Hammersmith description as the signal operator at barking and whitechapel can not put a circle description up
It's a mixture of what we on the signalling side can put up, and not what you can put on the front of the train. For example, at Baker Street we have many descriptions we can put up - but whether or not the next control room/cabin will have the correct description transmitted to them is another matter.
Therefore, there are quite a few "standard" substitutions for trains heading a certain way. For example, passenger train heading from Whitechapel to Liverpool Street will normally be shown as Hammersmith (as already mentioned). Passenger train heading towards Tower Hill and going to a non-standard destination will generally be a Putney Bridge.