If you can get from downtown Brussels to London (not Luton, Stanstead or similar) either for 30 Euros, or in 30 minutes (your post is a bit ambiguous), then I'd very much like a link.
Traveling in from the outlier airports is not exactly free, or instantaneous, from my experience.
Add to that, the train-ride on Eurostar is more comfortable (can work on laptop, has power-outlet etc), has better service, and less security-theater and hurry-up-and-wait, I'll take the train ca every time.
Note: I just dbl-checked - seems the cheapest train is now 43 Euros one-way, making it 86 Euros return.