SpeedFerries £50 return, Sea France almost certainly undercuts P&O, but I would go with P&O every time. I've never tried simply turning up to get the next available ferry but I'd guess that must be the cheapest - short of swimming.
I've had a think about it - and i think speedferries maybe the fairest priced, but i'll do some more research, and if you couldjust hang-on a few more days, i'll have the definitive answer