Just doing a few 'back of the envelope' calculations, here.

Matt Hancock said last week and also today on the Andrew Marr Show, that there is around 1 person in a 1000 infected at the moment. If that is true, and it is also true that there were 'tens of thousands' in the marches, it is reasonable to assume a few tens of people infected in the march. There are problems with that assumption, because it assumes a uniform spread of infections, but it will do as a starting point.

A protest is an unusual situation, because of close contact with a lot of people you don't know for a long time, but it is also outside. The R rate will not be the 1-ish of the lockdown, nor will it be 3 to 4 of 'normal circumstances': I would guess it may be several times higher - 10 or more.

So we have a rough guess that this might cause say 300 additional infections initially. If they have an R of around 1 when they get home, this is no worse than the infection rate around the end of May. (June 5th reported as 1557 per day, 2095 on May 29th)

So not negligible by any means, but it should be manageable. More of an unknown is how this is altered because we are relaxing other restrictions at the same time. Of the two, I would expect the relaxations to be more significant simply because we are talking of around 70 million people rather than a few tens of thousands.