You could take EasyJet
and fly from London's Luton airport to Paris Charles De Gaulle (main airport) for anywhere between 10 pounds and 40 pounds or so, plus tax. That's what I'd recommend, if Paris is your preferred destination. There are ferry crossings from Dover to Calais and they're not too expensive, but you have to get to Dover and then get to Paris or wherever you're going from there.
Or give EuroLines
a try-- take a coach. It'll be a much longer journey though.