Not really, since he MAY have 60 ppm FC, with very high CYA. OTO will show dark orange under those conditions, but DPD will bleach out even at a 3:1 dilution. So he could use syringaldazine strips -- which do NOT bleach out -- as a present / absent test for chlorine, so he can tell whether a DPD zero result is a zero low, or a zero high, and work up a dilution that way. But by the time you dilute DPD that much, you'd be better off with OTO, if it's available.

Of course, it may not. The UK is somewhat notoriously more paranoid about work place dangers than the US. (There've been some stories recently about firemen unable to perform rescues because of extreme workplace safety restrictions, resulting both in unnecessary deaths, and in embarrassing situations where bystanders performed rescues as in the presence of 20+ fire and rescue vehicles and dozens of rescuers who sat on their hands!) Anyhow, OTO has been listed off and on as a possible carcinogen (if you drink it, or bathe in it!) so it may not be available; I don't know.