It's reasonably major as it potentially involves multiple distinct parts of the government, several cabinet level officials and there will always be questions about could it be terror related, or could it impact the UK aviation sector.
IIRC there have been COBRA meetings over other transport incidents in the past, as it's basically meeting of senior officials (usually in a specific room which is how it got the name), some of which were a lot less complicated than this (the fact this happened in another country complicates things a lot).
Given there are potentially hundreds of people in the UK affected, many of whom might need to go over the India to deal with the deaths of relatives (and possibly bring bodies back) it's a fairly complicated thing to deal with on this end, that could involve a lot of Police forces (to give the death notifications), visas, repatriation of the bodies (either commercial flights, or possibly a charter) etc.