Triumph had virtually completed development of a 250cc single, ostensibly for Asian markets, back in 2013 (see pic).
They got cold feet when they realised they had nowhere near the production capacity that would be required if it was a success. They looked very closely at the KTM business model for the 125/200/390 single cylinder Duke series and decided to follow in their (seemingly successful) footsteps. Partnering with Bajaj must have required intense due diligence and some delicate wording of the contractual agreements so as to protect each parties commercial interests. It may all be for nought with Bajaj though as Piaggio has found to their cost over the years.
I guess they had little other choice of partner though. Maruti still has joint ventures with Suzuki, The Honda /Hero association continues quite strongly and LML don't have the four stroke engine building capability that would be necessary. I bet they looked closely at Royal Enfield though and it'd be interesting to know who ultimately emerged as the unwilling partner in that idea gaining traction.