The ECJ is part of the ECHR framework, not part of the EU framework. So by remaining part of the ECHR, the UK is still -technically- bound by the rulings of the ECJ.
The rub is, that the ECHR has no enforcement mechanism by itself, so the UK could just choose to ignore any rulings of the ECJ it doesn't like. It was the EU treaties that made adherence to the ECJ rulings mandatory, and therefore formed an enforcement mechanism for the ECJ.
No, the ECJ is the highest court of the EU. You're confusing the European Court of Justice (EU institution) with the European Court of Human Rights (Council of Europe institution).
The rub is, that the ECHR has no enforcement mechanism by itself, so the UK could just choose to ignore any rulings of the ECJ it doesn't like. It was the EU treaties that made adherence to the ECJ rulings mandatory, and therefore formed an enforcement mechanism for the ECJ.