Natfhe is among five unions, representing 750,000 members between them, which have called on the Trades Union Congress to meet to discuss its response to the likely conflict.
Mr Mackney, general secretary of Natfhe, said: "They (ministers) say they can't find more money for colleges but seem to have no difficulty finding billions of pounds for war. I don't want to be general secretary if it means I can't say what I think when the Government has got its priorities wrong."
Natfhe's national banner will be at the Victoria Embankment assembly point, where members will be arriving at noon.
Northern members are expected to congregate at the Gower Street assembly point at the same time.
Representatives of American trades unions will also be attending at the demonstration in London.