The main page on phasing show that when a double-circuit overhead line has transposed phasing the fields are lower to the sides of the line than if it has untransposed phasing. So can overhead lines always have transposed phasing? The answer is, they usually can, but not always.
The order of the phases on an overhead line has a small effect on the impedances - one phase is nearer the ground than the others. These different impedances can create zero-sequence and negative-sequence currents and voltages. These can't be allowed to get too big or they can cause damage. So, occasionally, the phasing of a line may be required not to be transposed for these system reasons.
Consider three overhead lines A, B and C meeting at a "T" point (in practice they would meet at a single pylon but we've drawn it slightly expanded to make it clearer).
Let's start with the right-hand circuit of line A. This has red phase at the top. Therefore, line B also has red phase at the top on its left-hand circuit. To make line B transposed, we put red phase at the bottom on the right hand circuit. So line C has red phase at the bottom of its left-hand circuit and has to have red phase at the top of its right-hand circuit to keep it transposed. But this means line A has red phase at the top of both circuits - it is untransposed.
So, when three lines meet at a "T" point, you can have two of them transposed, but not all three. This is the commonest reason for untransposed lines on the National Grid system. (You can get all three transposed if you insert a "phase-transposition tower", an extra tower which swaps over the order of the phases, but this is a big and visually intrusive structure.)
In some countries, though not in the UK, they deliberately have all lines with the same order of phases, so every line is untransposed. This is so that line workers know which phase is which without having to look it up. It is said to be a way of reducing accidents.