NEXT Saturday's antiwar march and demonstration in London could hardly have been better timed. It will take place just a few hours after the crucial Security Council meeting in New York on Friday which could well be the beginning of a sequence of events leading to war against Iraq, with or without United Nations authority. The size and representation of the march will therefore deliver an important message to Tony Blair as he faces the prospect of having to support the United States in an illegal use of force. Since the Prime Minister has effectively prevented the people's representatives in Parliament from expressing their opinion on the appropriateness of military action, the people themselves have to make their views known in the most direct way open to them. The organisers of the march believe it will be one of the biggest demonstrations in London's history, certainly equalling last year's Countryside march which was said to number 300'000 people; it is expected to last six hours, including the final rally in Hyde Park. There are two routes, one starting in Gower Street and the other on the Thames Embankment; they will merge at Piccadilly Circus and continue to Hyde Park. In New York a similar march planned for Saturday and due to pass the United Nations building has been stopped by a court order following police intervention on safety grounds; instead a stationary rally will be permitted but the organisers of the event, United for Peace and Justice, who expect it to draw at least 100'000 people, have protested that their Constitutional rights have been violated.