Thank you for your support.
It appears that one has to enter & exit India within 3 months. eg. If UK Citizenship Certificate date says 01/05/14, one has to enter & exit before 31/07/14
We will just take Citizenship Certificate and apply for UK passport on return, after India trip.
http://www.cgihk.gov.in/surrender_passport.php
3. A grace period of three months is allowed from the date of acquisition of foreign citizenship, for the purpose of travel on Indian passport. Therefore, no penalty will be leviable for travel on Indian passport within three months of acquisition of foreign passport.