Oreo
Marches don’t influence policy, particularly foreign policy.If you think about it, it’s the government’s job to decide on that and not popular public opinion.
The same goes for State visits where diplomacy is the order of the day.
The only marches or protests that may influence policy are on domestic matters where it becomes obvious that there’s a real problem and trouble brewing for the Party in power.
I'm not that knowledgable on the period, but one of the reasons Nixon got out of Vietnam was surely because of the unpopularity of the war at home, especially with people of draft age.
There were lots of anti-war marches which may have influenced foreign policy in the USA at that time. Anyway, what else, once you've written to your MP & received an anodyne answer, can you do?