It seems that Eurostar is expensive and inconvenient unless you live in Central London.

I've never seen a return fare at a decent time for less than £120.

In contrast there are plenty of fights to Paris from London airports for around £80-100 return, sometimes even less.

In addition to that airport parking is much cheaper than St. Pancras and Central London parking.

There is pretty much cheap parking in Central London, so you would have to take the train from anywhere outside of London, which increases the cost of the ticket even more.

Basically I don't see how Eurostar is able to compete with airlines because unless you live in Central London, I can't see any circumstances in which taking the Eurostar would be more efficient and more reasonably priced than flying.