Obviously I don't know where in the UK you are so here are a few examples:
From London, Eurostar will easily get you to Paris in three hours, including check-in, security and border control. The thing with Eurostar is that as long as you join the appropriate check-in line by the 30 minute cut-off (20 for Premier class) you'll get on the train. If there are delays in security or border control, the train will wait for you. Airlines don't do that.
From Manchester, 5h45 according to DB (though I've cut 20 minutes off of that time before by allowing less time to get from Euston to St Pancras and check in).
From Bristol, 5h40 according to DB (again I'd confidently shave 30 minutes from that time).
So how long does flying take? Well, firstly the airport probably isn't conveniently located. Less of an issue for London as there are several to choose from. Even so, more people will live closer to St Pancras than to their nearest airport, given that Gatwick, Luton and Stansted aren't actually in London. Bristol Airport is in the middle of nowhere whereas train passengers have a choice of two main stations, one in the centre and one just off of the motorway. Manchester Airport is on the south side of the city, whereas there are three stations in Greater Manchester with fast services to London (Piccadilly, Stockport and Wigan North Western) so the number of people who live closer to the airport than to a railway station are in a small minority. So if you live in Bolton or Rochdale, it'll take you twice as long to reach the airport as the railway station. If you're in Sale it's about even.
Then you have to allow time to get through security. The airlines generally seem to say a minimum of 40 minutes - but if there are security delays the plane will take off regardless, just look at what has been happening to people caught up in EES delays. In practice the airlines recommend you arrive around 2-3 hours before your flight.
Then flight time. From various London airports it's 1h20 to CDG, 1h35 to Orly. From Manchester it's 1h20 to Beauvais, 1h35 to CDG. From Bristol 1h20 to CDG.
Now you've landed, you need to go through passport control. EES is currently being implemented so it could take you anything from 90 minutes to 3 hours to get through. No such problems on Eurostar, you just walk out of the station.
Then you need to actually get to Paris. Not an issue with Eurostar, you're already in Paris. From CDG on the other hand, 40m. From Beauvais, 1h15. From Orly only 20m, but only one airline offers that route.
So, from any of the UK airports it'll take you at least 5 hours door-to-door. For Londoners this means that the train is far superior to flying. For Manchester and Bristol flying might be quicker, but only just.
So what about the cost then? What's actually included? Eurostar will let me (should I wish to) take two full-sized suitcases and a handbag onboard at no charge. The low-cost airlines by contrast make their money by stinging you for these extras, so unless you can live for a week out of a small purse you'll need to pay more than the headline fare.
Competition is coming to the Channel Tunnel too, with both Virgin and Trenitalia looking to take a chunk out of Eurostar's monopoly. That'll force Eurostar to drop fares.
As for the passenger experience, I'll take Eurostar over Ryanair any day.