Eurostar has had sporadic special offers, with return tickets going as low as £59 return to Paris/Brussels. Aside from these, it appears to be offering an ongoing £69 return fare from London to Paris. These £69 returns are incredibly easy to search for on the Eurostar site, so it's worth having a look. Each to their own of course, but if faced with the choice of train or plane from London to Paris, I'd take the train every time.
