I completely forgot to reply to this one. As it happens it makes no difference for this year as the boaty posts are all filled, but the best (and only thing to do) is to keep an eye on the website at British Antarctic Survey - Homepages
There is the "Antarctic Employment Pool" which is made up of previously employed skippers which sometimes fill the posts, but if no one is available it goes external and gets advertised on the website. There may be other interesting jobs for you there - for instance as a Marine, you would be qualified to go as a mountain leader (what they call a general assistant!) basically looking after groups of scientists in the field for extended periods leading them across mountains and crevassed ice caps and helping them put tents up etc...