I don't know whether or not the Seedcamp T&Cs are legal.

But, they are logical IMHO. You get about 25 hours worth of advice from Saul's high-value network of entrepreneurs and investors. In exchange for this advice, Seedcamp gets an option to invest at a reasonable valuation given the stage of the companies that attend.

Rather than make this option a legal Security in your company, they take you at your word. Seems better than wasting everyone's time with legal fees to actually formalize this option.