Dear specialists
Please, correct/comment following:
"To comply to BS En 600079-10 Electrical Apparatus for Explosive Gas Atmospheres, cable entries to the junction boxes and cable glands shall be metric (not imperial units of measurement."
First of all, BS EN 60079-10 is about hazardous area classification, not the selection of equipment for hazardous areas (which constitute some of the other parts of EN 60079).
I'm not familiar with the British Standards, but I presume that they are a harmonisation of the IEC 60079 series, which is what a few countries have done and which amounts to a copy and paste of the IEC standards (possibly with some ammendments). So I'll give you and IEC perspective.
From my reading of the standards, I don't see anywhere it is a requirement to use metric threaded cable glands / entries. Generally (but not in all cases), the gland just needs to be certified for the hazardous area application.
The difficulty may be in finding an NPT gland that is certified to IECEx or whatever HA certification you need. Furthermore, boxes certified to IECEx will most likely have metric threaded entries.
But in principle, I don't think there is anything in the IEC standards forbidding you to use NPT glands.