I need to book tickets Chennai Paris London Chennai, but am finding that the multi city return ticket way more expensive than two single legs onward Chennai Paris and return London Chennai. However, I was told at the airline office that not booking a round trip ticket might pose problems with immigration. We have a UK tourist visa and are applying for a Schengen tourist visa to France. Is that likely? I will have my return ticket with me - only it would be issued as originating from London rather than Chennai. Total travel time is about 40 days.
The main concern for immigration is "will you go back, as claimed?". Anything that causes them to doubt that is to be avoided. Here you do introduce a slight bit of doubt for the UK as you're missing an exit ticket for the UK. Once you do have an Eurostar ticket plus airline ticket from Paris, you have a convincing story how you'll leave the UK. The French won't care as much that you're still missing an entry ticket because as far as they're concerned, your return ticket is settled.