One of those recent posters realized the best thing to do was stay overnight in London, near St Pancras, and take an early train the next morning.
There are two ways to get from the airport to the station [1] by tube, which may include going up/down stairs with your luggage and could be crowded with rush hour travelers or [2] by driving (shuttle, taxi) which is likely to get bogged down in rush hour traffic and could take longer than the tube. [1] takes about an hour once you've walked to the Underground station, purchased your tickets (presumably with local currency), and are on the platform.
At St Pancras, you have to go through security, then through French passport control. They close well before the train departs, so there is ample time for all passengers to board the train. It's like getting to the gate after they've closed it. There may still be a good few minutes before departure, but you won't have access.
As already noted, you have no way of knowing what delays there may be. The plane could be late because of bad weather anywhere on the route, holding pattern to land, waiting on tarmac for a gate to open to name a few. There is no way to predict the length of lines at immigration. If you checked luggage, you could have a long wait for it and if it somehow gets delayed . . .
Of course, it's possible that you breeze through and get to St Pancras by 5.30, in plenty of time for the 6.30 train. How big a gambler are you? You are betting that you won't lose the price of the cheap ticket you bought in advance. The price for the 7.25 and 8.00 pm trains today is £243. I'd rather spend half that for a hotel for the night. If you are traveling with a companion, you'd be shelling out £486.