Entering Japan without an onward journey

Would having a Working Holiday Visa allow me to do this?

I'm unsure. The reason I ask is that the cheapest return I have found is £515 and the cheapest one way is £345.

Yes, I don't believe WHV holders (and resident visa holders) are required to have an onward connection.

Japan does not have any specific requirement for onward travel as part of entry procedures. For some reason some airlines do require this proof though, but not normally for citizen, residents, and long stay visa holders. You should have no problems.

Thank you very much for the responses.

(The following is a general remark, don't take it personally. For you, you will be fine.)

It is true that having an onwards ticket is not strictly required, in the sense that not having one doesn't mean you are automatically denied entry. It could be a red flag, however, indicating that perhaps you won't really leave as you're supposed to. If you have other red flags (say, you come from a developing country or you have already overstayed in the past), it could be the last straw.