Same questions as above.
What type of pylons?
How many and what is the weight of them?
Metal, Wood or other??
When slinging powerline-poles of wood, we allways fly them with one roundsling with a double choke at the top of the pole, secured by a bolt or some 4-5 inch nails on top of the sling, to prevent it from slipping (really not a problem unless you're encountering turbulence) Having had poles up to 15 meters weighing up to 1300 kgs, nice and stable cruise. That would not happen if you're flying it horizontal.
Same thing flying metal beams, that does not need to be mounted right away.
Bundles of rods horizontally, regardless how many, tend to swing back and forth and might be very tiresome, for 1,5 hour flight.
Being 1,5 hour from anywhere, I curious to where this might be?? There must be roads much closer to any point in the UK, unless you're going serious over water with them.