It sounds to me your itinerary was LHR - Dubai - Kochi. If this was purchased as such, it seems to me your contract was to get to Kochi and the planned change in Dubai is immaterial. You arrived more than 4 hours late, your departed from the EU and there were no extraordinary circumstances. Seems simple to me and bear in mind pretty much all airlines will initially reject claims.
Push back on them and if necessary use the usual methods to get them to pay up.