Seeking advise on legalizing a U.K marriage in Kerala

Hi, please bear with me as I explain this situation. My sister is a 31F Malayali, Marthomite currently based in the U.K. She met and fell in love with a British man while there, and is due to enter a civil marriage in the U.K this month- very much approved by both the families. However, we are sort of in a dilemma as our family, esp. grandparents, in Kerala would like to see her get married in our church. Due to visa requirements, it is imperative that they get married in the U.K this month (please, not a sham marriage, it is a genuine relationship of almost 3 years- I visited them in September in the U.K, they are truly one-of-a-kind couple). Could I please seek some advise on how she could legalize/recognize their marriage in Kerala after entering the civil marriage this month in the U.K, and are there any Marthomites (or anyone here) who would be able to advise if a church wedding is possible in a year or two? Are there specific requirements we should be aware of, especially since her fiancé is British and not currently baptized in the Marthomite faith? Because I know the woman, she's not planning to revoke her citizenship as our parents will be here, so I guess, she wants to have the relationship legalized in India. And they want to find the money for the wedding (hence the two year wait). Any advise or suggestion is welcome. Thanks, and Happy New Year!