Back to the general question. Previous posters are correct that overnight bus travel is common and safe. A few things about this particular journey are worth remembering.

The departure point in Paris is outside the city center and will be a hike to get there. Those intermediate stops will be a real pain when you're trying to sleep in the middle of the night, as will the RoRo of the Ferry. Plan on missing a night's sleep.

Victoria Coach Station is fine and a quick walk to Victoria Station itself which will give you entre into the tube system. Many bus routes also converge on the area.

I'd have to agree that unless it is tons more money more the way to go is by the chunnel.