"four main carriers (O2, Virgin, Three, and Vodafone)" - think that should be "O2, EE/BT/OpenReach, Three, and Vodafone".
Regarding backhaul, EE/BT/OpenReach and Vodafone (from the Cable and Wireless acquisition) are already in the position to exploit the price hikes that are feared from o2/VM. It seems a bit ridiculous to be worried about anybody other than EE/BT/OpenReach doing this, given the scale of BT position in the UK compared to the other operators.