I am an Indian planning to take a train from Disneyland Resort (Marne-la-Vallée/Chessy) to London. Please let me know where France and UK immigration respectively will be done.
If you are taking the direct train (daily during the Summer and other high-demand times, five times weekly the rest of the year) from Marne-la-Vallée-Chessy to London St. Pancras, then French Immigration Exit and UK Immigration entry are performed in Marne-la-Vallée-Chessy station - to be precise, in the upper concourse of that station.
Because the station only does this once a day, the queues can get pretty long. Eurostar generally recommend checking in an hour before departure, and I'd support them on that: they don't have the sorts of staff numbers that there are in Gare du Nord or St Pancras to process lots of passengers quickly.
It'll be similar to flying: French immigration is performed at Marne la Vallée Chessy, while UK Immigration takes place on arrival at St Pancras.
This is the only Eurostar route where UK immigration is not cleared in mainland Europe.