There is a sort of single rider line.
Once you get to the point where the FPP line merges with the SB line, you are sorted into one of six lines (three on each side of a dividing wall) though typically only four of them (two on each side) are used. The centre line is often used for single riders.
I first noticed this when the CM at the second FPP touchpoint assumed I was with the group in front of me and directed me to the left-hand line, which was quite long. As I waited I saw single people joining the middle line and moving to the first pre-show room almost immediately. I returned to the touchpoint and asked the CM about it. On realising I was a single rider, she directed me to the middle line and I was called to the pre-show room within about a minute, having walked almost the full length of the other line straight to the front of the new one I’d been directed to.