I've not really decided yet. On 400k+ rides I normally carry a very light (600g) sleeping bag pre-fitted into a breathable bivvy bag. That squeezes into my Nelson Longflap or straps on top in a dry bag with the longflap in shortflap mode. If I'm actually intending sleeping somewhere (like when I ride a couple of hundred K to the start of a Welsh ride and back) I'll take a thermarest-alike, but that means an extra bag on the bike - usually a pannier as that way I have room for some extra clothes. I really don't want a pannier on LEL.
For LEL I'll definitely carry at least the bivvy bag on the bike (takes almost zero room, weighs very little) just to satisfy my need to feel I have shelter if things go wrong in the middle of the night, in the rain, in the middle of nowhere. For the sleeping bag I'm torn between carrying it, or putting one in each of my bag drops. Carrying means my own bag wherever I choose to kip, which is more flexible, but obviously means lugging it up and down the country.