In a sane world the UK would indicate a desire to leave the EU, would then try to negotiate an exit and then Parliament would pass a bill to ratify to the treaty. Parliament will still need to pass that Bill - but now we're being told they need to pass a Bill just so the government can open negotiations.
To be frank, mere executive discretion to negotiate something under very broad legislative approval should not be a Constitutional question, certainly not at this point in your country's history. If it indeed is, then that needs to be checked one way or another by the courts.