Bath for Europe is not running coaches to The People’s March 4EU on 09 September 2017. You will need to make your own arrangements. London is likely to be extremely busy, as the march coincides with Proms in the Park, at Hyde Park and Last Night of the Proms at the Royal Albert Hall.
For cheap train deals try Trainline or for coaches National Express not forgetting to work out whether its worth buying a discount card. With a an adult leisure railcard, you can take other people with you. Or there are group discounts for 3-9 people on GroupSave discounts from Great Western Railway and South West Trains for example.
If you register for free with the Bath for Europe Eventbrite page you’ll get informed of their meeting place in Hyde Park.
If you have tickets for Proms in the Park, that starts at 1500 BST in Hyde Park and the rally at Parliament Square is due to finish at 1530 BST.