TTIP has to be agreed at the EU level. Even if the UK left the EU, it would not matter for TTIP specifically (it may for a different agreement, of course).
The Human Rights Act is a UK domestic bill that gives primacy of the European Convention of Human Rights directly into UK law, so that people do not have to go to the European Court of Human Rights outside the country every time they want to use the Convention as the UK judges would have to act according to it.
The current UK government want to scrap that in order to force people who want that defense to have no direct recourse in UK courts, only in Europe. It gives them a way to implement the dodgiest possible laws across a huge range of topics without them being immediately shut down by their own UK judges for infringing human rights. It also means once decisions are made against the government at the European level, it would now be the current government choosing whether and how to implement the judgement, instead of the UK justice system having the power to do it directly and automatically.