According to The Guardian 16th April 2017, an attempt to bring a private prosecution of Tony Blair for the crime of aggression was rejected by a District Judge (Magistrates' Courts) at Westminster Magistrates' Court. The judge refused to issue a summons but, as far as I can ascertain, the reasons of the judge for doing so were not published. The Guardian report goes on to say that the present Attorney General - Jeremy Wright QC - is involved in proceedings in the High Court in which the District Judge's decision is being challenged. The Attorney seeks to stop the prosecution and the basis for doing so is that the crime of aggression is unknown to English law.
The crime of aggression was included in the remit of the International Military Tribunals which sat in Nuremberg and Tokyo after the conclusion of World War 2. It met with strong criticism at the time concerning breach of the principle of legality (nullum crimen sine lege). In 1946, the United Nations endorsed the crime of aggression in customary law but the definition of the offence remained contentious. When the International Criminal Court (ICC) was created , the crime was included in its statute but the statute also stipulated that the jurisdiction of the court would not be exercised until a generally accepted definition of the crime was adopted. The definition was finalised at Kampala in June 2010 - Watching the Law 4th September 2012. The law will not be applied retrospectively.
Article 8bis - Aggression
Attempts to recognise aggression as an offence in English law were rejected by the House of Lords. The decisions in R v Jones and others  UKHL 16 and R (Gentle) v The Prime Minister and others  UKHL 20 are crucial reading.
In his advice to the Cabinet (7th March 2003) Lord Goldsmith referred to the possibility of a prosecution for aggression. The advice stated that - "Aggression is a crime under customary international law which automatically forms part of domestic law."
The conclusion must be that no prosecution for aggression is possible either before the ICC or in the UK courts.
The Chilcot Report was the work of a committee of Privy Counsellors. It was not a court of law. The Report was issued on 6th July 2016 and avoided making any statement that the Iraq War was unlawful - Law and Lawyers July 2016 - though the report concluded that "the circumstances in which it was decided that there was a legal basis for UK military action were far from satisfactory." Chapter V of the Chilcot Report examined how legal advice from the Attorney General (Lord Goldsmith QC) was made available to the Cabinet.