My research confirms Methyl's findings that KLM CityHopper is the only direct flight operator. For a random date I've chosen (Wed 16 July) all direct flights are priced at €595. (KLM CityHopper is a business airline, so don't expect anything cheap!).
However the Deutsche Bahn website (which integrates
all European rail timetables) is finding rather quicker rail journeys than Methyl suggests. The departures at 0618 and 0818 both take 5h 42, changing at Brussels Midi in each case. Journeys requiring a cross-Paris transfer take approximately 6h 40m (with a very generous 70 min allowance for the transfer between Paris Nord and Gare de Lyon - it's just two stops on RER Line D):
http://www.bahn.de/i/view/GBR/en/index.shtml
While I like the Deutsche Bahn website for researching train times, it often can't provide fares. SNCF is better for that. I've been finding single fares of €144.50 or €159.50 via Brussels or between €113.00 and €166.00 via Paris:
http://uk.voyages-sncf.com/en/
The excellent ViaMichelin route planner estimates the driving time as 9h 40m and the total cost of tolls (€30.10) and petrol (€104.79) coming to €134.89. That's roughly the same as the train fare even before you add in the cost of hiring a car!
http://www.viamichelin.co.uk/
I'd take the train ;-)