Under US Law, (the Logan Act) it is illegal for sitting representatives of the Federal Government to attend. However, so far as I know, there is no such law in the UK:
Marcus Agius, Chairman, Barclays plc
Nick Boles, MP
Kenneth Clarke, MP, Lord
Chancellor and Secretary of Justice (regular attendee)
Robert Dudley, Group Chief Executive, BP plc
Douglas J. Flint, Group Chairman, HSBC Holdings plc
John Kerr, Independent Member, House of Lords
Peter Mandelson, Member, House of Lords; Chairman, Global
Counsel (regular attendee)
John Micklethwait, Editor-in-Chief, The Economist
Gideon Rachman, Chief Foreign Affairs Commentator, The
Financial Times
Peter Voser, CEO, Royal Dutch Shell plc
Martin H. Wolf, Chief Economics Commentator, The Financial
Times
Vendeline von Bredow, Business Correspondent, The Economist
Will any of the above Members of Parliament be questioned about their attendance, since it is an "Act of Treason?"