I don't see why the claim would fail. I actually read the entire booklet from HMRC on this matter, available here https://www.gov.uk/government/publications/490-employee-travel-a-tax-and-nics-guide
A lot of people seen to hang to the fact that the expense might be considered unnecessary. The thing is, you just have to read the booklet to see that the necessity test is not about the expense being necessary but all about the trip being necessary. HMRC doesn't care if you've taken a £1 Megabus to Edinburgh or a £700 full fare J ticket.