Another user here, still thinking of a good name. I went on the march.
Great things:
Sunshine, numbers, mound of beautiful flowers in memorium at gates of HOC, great speeches, nice people, hipster police, sound system in Whitehall.
There was a scary forecast of what could be round the corner for us by the Polish speaker: as government is becoming more totalitarian in Poland, judges are chosen for political expediency, civil rights are shut down and surveillance increased. That all sounds very like what May is doing now.
And optimistic appreciation of what liberalism joining EU meant for Poland. It was interesting.
Lammy was outspoken calling May's regime totalitarian. He was also honest and right that the brexit campaign was racist.
The train ride back was a chance to link up with local remainers; the fight goes on. 
Thanks for the thread.