What's the reason for this unusual transmission tower (pylon) design?
It appears from your photo that the lower lines have been raised to clear the roadway below. 


Note that the lower lines require two insulators per phase whereas the upper ones only require one. There is a cost saving to only applying this style to the lines that require it.
The lower lines also require two splices - one at each end of the loop - whereas the upper ones can be clamped by the fixing at the bottom of the insulator. I imagine that for installation all the wires are drawn through pulleys, transferred to clamps and the bottom one clamped by the horizontal insulators before being cut and looped.
Each of the horizontal insulators on the bottom lines are approximately the same length as the ones on the upper lines. This is to be expected to maintain breakdown voltage between the lines and the earthed frame of the pylon.


You may be interested to know that Britain has a Pylon Appreciation Society!



Figure 1. A simple straight-through clamping arrangement. Notice the Stockbridge or tuned-mass dampers each side of the insulator. Source: Wikipedia Commons.



Figure 2. The horizontal insulator arrangement is more commonly seen on turning pylons. Source: Pylons.org.



Figure 3. "CHECK IF THE LIGHTS ARE WORKING NOW!" An insulator in tension.