The question was posed at a press conference of Foreign Secretary Vijay Gokhale in London.
"Eight of the nine people whose tickets were allocated for the event have not got their tickets. Why have the tickets been cancelled? Why are our voices being suppressed," asked a student.
"This is a press conference about India-British relations and the Prime Minister's visit to Great Britain. These are not the questions I can answer, said Mr Gokhale.
When the student pressed further, saying "at least we can be told why we haven't received", the Indian envoy to the UK YK Sinha butted in.
"Why are you pre-judging what the Prime Minister may or may not say in an hour's time? Why don't you wait? Please ask the people who are organising it. We will not be able to answer," said Mr Sinha.
PM Modi addressed the Indian diaspora at a townhall called 'Bharat ki Baat, Sabke Saath' at London's iconic is Central Hall Westminster.
"You have not shied away from taking difficult decisions in the past, such as with demonetisation. Please take similar extraordinary steps to prove that India's daughters matter," the letter said.
The students said they were looking forward to attending the event.
"When you arrive and address #BharatKiBaatSabkeSaath, you can tell us and the world what those extraordinary measures are that you are putting into place to show that enough is enough," the letter said.
Last week, PM Modi had said justice will be done and no criminal will be spared, referring to the rape cases that have sparked outrage across India.
"I want to assure the nation that no criminal will be spared. Justice will be done. Our daughters will get justice," the Prime Minister had said at an event to unveil the BR Ambedkar memorial in New Delhi.