visas – How to get one itinerary instead of multiple tickets?

I’m trying to travel to Nassau, Bahamas from Colombo, Sri Lanka via 1 connecting flight in London Heathrow.

How do I get this all on one itinerary? I can see there are flights in February on flightconnections.com but when I searched google flights and ITA, I can’t find that route on a single itinerary. Is it possible WITHOUT using a travel agent?

Basically, I want to satisfy the “onward” ticket requirement in the DATV transit visa rqeuirement. I think they will not grant it if it’s 2 separate tickets.