Tricky one, and I can see why X-ray was annoyed, but I'd have just added the 10p and said nothing
It's always nice to get a tip, even if it's only 10p or so, but I notice it more on the short trips. For example, for years the main student halls here were £4.95 on T2, and the vast majority of students would take the 5p
And late night in term time it wasn't that unusual to get maybe three or four of those £4.95 student runs on the bounce, and they'd all wait for the 5p
Another student hall from the station was £9.90 on T1 for years, and I recall only getting the 10p as a tip on maybe 1 in 5 or maybe even just 1 in 10 trips
I generally round down the longer trips as well. For example, £20.40 and I'd normally just ask for the £20. Which prevents counting out the change, and avoids the possible annoyance if I ask for £20.40 and there's zero tip
Of course, with the card payments it's a different dynamic, and I'm maybe less likely to round down, because there's no change to count out.
For example, a student run clocking at £5.15 I'd normally round down if it's cash, particularly with the annoying 5p, and because I'd know there's little chance of a tip anyway. But with a card payment it's obviously not an issue simply keying in £5.15 rather than £5.00, and the 15p will cover the charge.
Of course, and coming back to X-ray's run, some people are so, er, arithmetically challenged that
their perception of what they were tipping him is maybe a bit different to
his perception