Prices for hiring a coach for a day?

From where? Glasgow would be a bit more than Dover, for example.
What time do you need to be in Paris, and for how long? You might require 2 drivers.
Will there be alcohol? Is it a football trip? They are likely to be refused.
Mixed gender group? Age range? If the majority are under 25 then expect a boost in rates.
What area of Paris? Will it be a tour or drop and collect?
Is it going to be a Duty Free run?
Lots of factors. (edited)

Just use flixbus £27 return pp London to Paris