Deepest part of the Channel that area, close to the Hurd Deep. They will probably also find wartime munitions and low level nuclear waste which will lijely disrupt the search.
https://en.m.wikipedia.org/wiki/Hurd%27s_Deep
Alderney Hurd Deep radioactive waste 'not dangerous' http://www.bbc.co.uk/news/world-europe-guernsey-22198566