Not much difference
The mine landed on the mudflats.
The group sent out to look at it already had a good idea of what it was, and took brass tools with them (non magnetic).
So even if had been armed it wouldn't have gone off.
As shoeburyness was a major centre for gunnery training, and research into ordnance in general and new weapons in particular, it might as well have been gift wrapped.
In the 30's shoeburyness experimental establishment trialled the following (amongst others):
25pdr, 3.7 inch AA, 5.5 inch medium, 15 inch coastal.
In the war they trialled rockets, and all sorts of wild and wacky stuff.
The small arms experimental establishment was there as well.
After the war shoeburyness was one of 3 nuclear weapon development sites.
TSR2 prototype was on one of the ranges, being used for target practice.