I agree they cannot expand that much that fast, which was my second point. They wouldn't be very keen either way, as ISRO's primary objective is their research

I agree they cannot expand that much that fast, which was my second point. They wouldn't be very keen either way, as ISRO's primary objective is their research missions while there is some drive to have commercial operations it is not that important to them. However it was also possible that Bharti (and U.K. government ) could have pulled enough strings to get ISRO to agree, so won't have been that surprised if they had partnered.