Assuming this is a multicore cable and not a single, I would say bond both ends to earth. If it is a single core carrying AC, you can get image currents flowing in the armour which will reduce the thermal rating of the cable. In this case you should be looking at aluminium armour, not SWA, and considering bonding only one end to earth.
Assuming you're using a standard brass gland to make off the cable at your plastic box, we tend to use a normal banjo and take a bond of 6mm2 or larger as required from the bolt hole in the banjo to the earth terminal within the enclosure. Use a large washer to spread the stress from the nut on the opposite side of the box from the banjo or you will fracture the plastic. I'd avoid counting on the armour as CPC in this situation, so include a core as the CPC.
I've tried to summarise the practices typically employed at our site. They aren't necessarily written down in tablets of stone, but we consider them to be safe practice. I might learn something from this too.
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If we learn from our mistakes,
I'm getting a great education!