If you are British, your best bet is probably with the British program, which Paul and Drummy doubtless know more about than I do.
That said, because Britain has maintained close ties with New Zealand, Australia, Canada and South Africa, that may open up other opportunities for you as well. I know that New Zealand, Australia, and South Africa have stations in Antarctica. I don't know of any Canadian program, but Kenn Borek Air, which operates out of Calgary, Alberta, provides services to both the U.S. Antarctic Program and some private groups.
I expect that these national programs and the Canadian company favor their own respective nationals, so non-specialized jobs may be difficult to get with them. However, skilled slots are sometimes difficult to fill, and if you have a skill that one of these countries needs that may greatly increase your chances. I do know that New Zealand's Scott Base has personnel from England, Scotland, Australia, and Canada at present, and due to the U.S. program's close relationship with New Zealand there are almost as many Kiwis at McMurdo Station this year as there are at Scott Base, as well as several Canadians. Besides that, I know the U.S. program has hired or hosted personnel from Zimbabwe, Japan, Sweden, the Netherlands, and France.
In any case, you have a number of options, and the more skills you have the more options you have. Your skills with boats may be quite useful at one of the many stations on the Peninsula (which includes the USAP's Palmer Station).
If you are really interested, make your interest known to prospective employers early and often. There are lots of people who apply on a whim, not so many who are really serious and will follow through.
You are correct that seasons here are opposite from those in Britain. There may still be a slim chance of getting a job for this coming summer season (roughly October to February once you get further south), but it is late enough that you will probably be looking at the 2010-2011 season. Stations on the Peninsula may have different schedules, as personnel can get in and out year round.
Most jobs are seasonal, and mostly summer season, but expect to put in long hours while you are here.
If you are concerned about the size of the station, consider that there are stations of many sizes - from those with 4 or 6 people to McMurdo Station which peaks at over 1200 during its 5 month summer season. If you think you would have difficulty at a small station, you probably will - so find someplace a little larger, or with a shorter season.
Best of luck.