I was in 'batches' for the bridge, which meant the queue was stop-start from about the gardens at the foot of the London eye, and extremely slow until on the bridge. It meant that the bit along the Embankment by the Covid Memorial Wall at St Thomas Hospital was pretty tedious, and when I was there, there were only 7 portaloos for that whole stretch, but at least you could stand in another (parallel) queue for the loo just for a bit of variety.
There's a nice coffee truck along there (the one that's always there)
Plenty of loos in Victoria Tower Gardens, btw. The ordinary public toilets were open, plus lots of portaloos.
Marshals were lovely, and the one I spoke to (about passing on feedback that they needed more loos along the Embankment) was very polite and friendly. Others in passing were all friendly too. (I'd rather see them getting to use the VIP entrance as their 'pay' than parliamentary staff and guests - they deserve it)