Travel2be complaint: Company has changed the my connecting flight and insisting me additinal charge to resolve

Dear SirMadam,

I have bought below ticket (Booking number: 19161630) from Tavel2be.com

FLIGHT BA 423 – British Airways 10-11-2021
DEPARTURE: Amsterdam, NL (Schiphol Intl.) 10-11-2021 07:10
ARRIVAL: London, GB (Heathrow) 10-11-2021 07:25

FLIGHT BA 143 – British Airways 10-11-2021
DEPARTURE: London, GB (Heathrow) 10-11-2021 11:15
ARRIVAL: Delhi, IN (Indira Gandhi Intl.) 11-11-2021 01:15

FLIGHT UK 747 – Vistara 11-11-2021
DEPARTURE: Delhi, IN (Indira Gandhi Intl.) 11-11-2021 06:15
ARRIVAL: Calcutta, IN (Subhas Chandra Bose) 11-11-2021 08:20

It has the return flight as well from Calcutta to Amsterdam via Mumbai and London.

Surprisingly, the flight from London to NewDelhi scehduled on 10th Nov has been cancelled. Because of this, I can not catch the scheduled flight from New Delhi to Calcutta on 11th nov.

Now when I am checking with travel2be agent, they are forcing me to pay additional (30 euro) per person to reschedule the flight either on 9th Nov or 11th Nov. When I checked with BA airways, they told me to talk to travel2be as the ticket was purchased from travel2be

Now I have two queries –

1) Why should I need to pay additional cost for this trouble where I have not asked for cancelling or changing my flight.
2) If I pay the additional cost and if the flight again gets cancelled, then who will resolve the trouble again

Thanks,
Mr Sarkar