Identify this tuber like object found on beach
It is spongy because it is a sponge, specifically a sea sponge. There are quite a large number of them native to the waters around england. I would guess Craniella cranium based on shape and location but I am by no means an expert on sponges. 
Here  a cross section of a cleaned sponge 


and sediment laden potato sponges after a hurricane. 

 

you can see more images of collecting these here, http://www.vims.edu/newsandevents/topstories/archives/2011/irene_blobs.php

Here is a whole website dedicated to english sea sponges, http://www.aphotomarine.com/sponge_suberites_ficus_sea_orange_sulphur.html