It's in Surrey but there's a mobilisation centre/fort of the same vintage, now restored and in National Trust ownership at Reigate, although it's only open for guided tours.
"Built in 1898, Reigate Fort was never designed as a conventional armed stronghold. It was originally called a Mobilisation Centre, as its main role was to supply tools and ammunition to enable soldiers, volunteers and contractors to dig entrenched positions along a designated section of the North Downs." TQ257521
I had thought Halstead was armed in some way, if only because it is sited at the mouth of the Darrent Valley. The casemates also suggest, to me at least, that it was intended as more than a simple store