I agree with Pete, it looks like someone's old rock collection, not local to Kent.
It is very difficult to identify a mineral from a picture alone, but here are my thoughts on it:-
My first thought on the white mineral was calcite or barite.
Looking at picture 3 there are signs of cubic crystal structure, so then I thought fluorite.
However, there are no signs of cleavage which would be expected on a piece that has been knocked about a bit.
There is an irregular crack on picture 2, so now I think it may be quartz that had grown on a cubic mineral such as fluorite.
If it will scratch easily with a knife point then it is not quartz, so it's either calcite, barite or fluorite.
I don't think it is gypsum, it is too soluble to last long buried in a garden.
I think the dark mineral may be oxidised/weathered siderite or possibly marcasite, both iron minerals, hence the rusty brown colour.