I don't think they will file, and having read the policy of DCM Ltd, I don't think they are acting illegally. I think it is a sad policy though.This. Let's see if they're idiotic enough to file a suit.
Rather than ban all political and religious adverts outright, they could easily be selective in where and how they are displayed. The advert might be appropriate for a Christmas or Christian related film, but wouldn't be displayed during, for instance, Bollywood week. Similar judgement would be applied for political advertising.
The argument that one doesn't want to see the Lord's Prayer advert is a poor one. Just talk over it and ignore it if it doesn't interest you.