There will be more than that.
Early this morning central London was chock full of them. Based on my limited exposure there are far more than that estimate and then there will be this mornings waves into Euston/ Kings Cross.
God only knows where they will go to watch as everything is more or less fully booked by locals according to the news last night and There are no screens in any of the parks/ sites aside from Trafalgar Square and that is reserved for front line staff.
They are already p1ssed but may be really p1ssed off when they cannot even watch it.