score:4
To get a visa, you don't need a ticket at all. It wouldn't make sense, because often visas are rejected and then someone would be stuck with a paid ticket that they cannot use.
However, when you get a visitor visa, the government wants to be sure that you are going back, and when you are going back. You will be asked how long you will stay. That's an important question that they want answered. And when you arrive in the UK, someone will look at your ticket, and a return ticket is a good indication that you will be returning when you said you would. So when you say "I need to be flexible wrt return dates", the person at the border will say "in that case, I would prefer that you don't enter". They want people who know when they are going to leave.
Upvote:1
This would depend on your personal circumstance you provide to support your application, but generally speaking the Visa officer will need see evidence that you will exit UK after your stipulated visa duration. In addition to other documentation, your visa application would definitely be stronger if you have a valid return ticket.