It looks, to me, as if that is a connection post which contains gel-crimps joining two sections of cable together. The fact that it is adjacent to the base of the pole makes me think that it is just a joint enclosure in the D-side cable, before it then proceeds up the pole to the pole-top DP.
Many years ago, just post WW2, the GPO started to install PCPs. In the area of SE London (where
b*cat was born), almost every telephone pole had an asbestos-cement tertiary cross-connection point at its base. The TCP was a pillar, about four foot high . . . I suspect the object you have photographed is a more-recent manifestation -- that serves a similar purpose.
If you can take a close-up photograph then I suspect that
Black Sheep will be able to tell us that item's correct name and purpose.