Your numbers are way off. Fastest Manchester-London train is 2h (the 0700 Manchester stoping 0707 Stockport and 0900 Euston). I don't think there's one quite that fast on the way back. It's about 180 miles. London is a late city but I wouldn't call Euston "the centre", it's not even in the congestion charge zone. Plan on a good 30 minutes to get to your destination in London and 15 in Manchester.

Paris is not 800 miles, or even 800km.

You're right that a car is unlikely to be the better choice for Manchester-london when ignoring costs (£124 each way on the quiet trains)