The GWR (and subsequently the WR of BR) always used lower quadrant signals, as indeed did other companies, for example the Midland Railway. But as I understand it, the lower quadrant arms needed heavy cast 'spectacles' (the bit with the coloured glass in) to ensure that that end of the arm was the heaviest so that signals automatically returned to the horizontal 'danger' position if any connection between the arm and the bits at the bottom of the signal post broke.
As signals developed, the upper quadrant became favoured as the arm and the spectacles could be made from lighter and thinner metalwork, yet the arm would still return to horizontal if the connections broke. But with the lighter and cheaper metalwork the signals were less costly to make and easier to maintain.
All signals, upper or lower quadrant, were sited at a height where the driver and fireman had the best view on approaching them. The various railway companies also had different driving positions for various historic reasons, and the GWR locos were all driven from the right-hand side. So the signals on the GWR had to be placed where the driver on the RHS of the loco could see them.
But railways are crossed by bridges and at stations there are buildings and canopies which may obstruct the view of signals. So the signals may have to be placed at different heights to give the best view. Even today with colour-light signals they still have to have 'Sighting Committees' when resignalling is taking place to ensure that the signals are placed where the drivers of all sorts of trains have the best view.
Take a look at
www.signalbox.org which has a comprehensive history of British signalling and many examples of signals and signal boxes from pre-grouping to BR.
Hope the above helps,
Regards,
John Webb
