Looking at CAA data for various WCML and ECML destinations to Paris and comparing with London to Amsterdam and Germany, it appears that the daily market each way is:
Manchester + Birmingham to Paris : 800
All ECML from Glasgow to Paris : 400
London to Amsterdam : 3000
London to Cologne + Frankfurt : 1600
So it's clear that the priority has to be on Amsterdam and Germany, but one can maybe make a case for a daily train each way from Manchester with a stop in Birmingham. The market size is bigger than Eurostar's new route to Lyon and Marseille (600 each way).
What you are not taking into account is that where there are frequent air services, for example Manchester - Paris and Birmingham - Paris, those services are timed to meet the demand at various points during the day.
The passengers are predominately travelling on business, or connecting at CDG onto other flights; although there is an amount of leisure traffic at certain times of the year.
The aircraft types used are sized accordingly to meet the demand.
If one of the airline operators decided to replace 5 flights a day, with a single flight using a larger aircraft, it simply wouldn't work. The timing of the single flight would not suit a large proportion of the prospective market and many would simply take a different option, or not travel at all.
That's why there's a spread of services throughout the day.
Providing a single 800 seat train from Manchester to Paris would not suit most of these passengers either. It wouldn't be able to get to Paris early enough for some business travellers and would be far to early in the day for others.
As for the passengers catching onwards flights from Paris, there would be virtually no interest in such a train, as the convenient connections that routing via CDG provided, are completely eradicated. They would choose to fly to a different hub (e.g. AMS or FRA) and make their connections there instead.