I got a handful of serious (not from a diploma mill) specialized short courses with the word 'Diploma' on them. They were all company sponsored training.
The issue here is that 'diploma' in the UK seems to be understood as an academic level, which implies significant efforts to obtain (but less than a bachelor).
I would not like to appear as some kind of trickster, who claims to have a batch of academic credentials that happen to be faux. However, I also don't see how I could express this situation to the HR guy, who will probably just skim through the CV.