If you are connecting through Brussels you walk across the platform... The cross-over time between trains at Brussels is a couple of minutes
No, you don't, and no, it isn't. The Eurostar terminal in Brussels is quite separate from the rest of the station and you cannot simply "walk across the platform" in "a couple of minutes". When transferring from Thalys to Eurostar you have to go through the same check-in, security and UK immigration formalities as passengers commencing their journey in Brussels, with a recommended minimum connection time of 40 minutes.
Even though I also prefer rail travel to flying, the lengths you claim to go to to avoid flying into London sound extreme, to say the least.