Does this explain why all the Rules are void?
The judgment says the Rules and the Amended Rules are "in breach of sections 2 and 18 of the Competition Act 1998"
Section 2 of the Act:
Agreements etc. preventing, restricting or distorting competition.
(1)Subject to section 3, agreements between undertakings, decisions by associations of undertakings or concerted practices which—
(a)may affect trade within the United Kingdom, and
(b)have as their object or effect the prevention, restriction or distortion of competition within the United Kingdom,
are prohibited unless they are exempt in accordance with the provisions of this Part.
(2)Subsection (1) applies, in particular, to agreements, decisions or practices which—
(a)directly or indirectly fix purchase or selling prices or any other trading conditions;
(b)limit or control production, markets, technical development or investment;
(c)share markets or sources of supply;
(d)apply dissimilar conditions to equivalent transactions with other trading parties, thereby placing them at a competitive disadvantage;
(e)make the conclusion of contracts subject to acceptance by the other parties of supplementary obligations which, by their nature or according to commercial usage, have no connection with the subject of such contracts.
(3)Subsection (1) applies only if the agreement, decision or practice is, or is intended to be, implemented in the United Kingdom.
(4)Any agreement or decision which is prohibited by subsection (1) is void.
So - City have shown that the Rules offend subsection 1 so the agreement is unlawful and prohibited (not just individual rules) and (subsection 4) if the agreement is prohibited it is void (the whole agreement - or the "decisions by associations of undertakings" if the Rules do not constitute an agreement).
The section means that anyone can take action against an "association" and argue that it's a competition-stopping cartel. In this case, one member of the association has done it!
Section 18 prohibits abuse of a dominant position.