In reply to Boulderdash86:
Dover to Calais is about all that's left, there are no ferries from Folkstone or going to Boulogne anymore. Newhaven - Dieppe still runs but is not a good bet, unless you lived in Newhaven possibly - more expensive and longer crossing. Some ferries go Dover to Dunkerque, but it's a longer crossing too - for vans and trailers it used to be cheaper.
I always take the tunnel these days as I buy a 10 trip voucher which works out cheaper, but for a single trip the tunnel is usually more expensive. The cheapest varies so you just have to check out the ferry lines, there aren't that many, Eurotunnel, LD lines, P&O... a quick google is required as there have been a few changes of late - Sea France has gone under, Speeferries too a while ago, a pity as it was the cheapest.
Whatever you do book and buy in advance, turning up and buying at the port is much more expensive... again years ago this was quite standard but internet has changed all that.