The Concorde briefing card actually admits that the seat pitch may be less than the upper body length of the passenger. Very few modern safety briefings address this point; BA's safety briefing video portrays some fantasy wonderland where everyone has Club-like seat pitch (the new one's no better).

I continue to remain unclear how to brace myself in almost any economy seat, based on the briefing and documents provided, on most airlines.

I am assuming that since the aim of bracing oneself is to avoid flying forward into things with a sudden impact, that leaning against the seat in front is a reasonable thing to do, and a very few briefing cards I've read do show this.