Least expensive way from Dublin to London.

Fly. search for discount UK airlines.

And be prepared to pack all of your belongings in a small gym bag - or pay big $ for overweight or extra luggage.

"Discount" airlines on this route are rarely any cheaper than legacy airlines.

Aer Lingus flights to Gatwick in particular consistently match Ryanair. Apart from Air France flights to London City (and evebn they can offer excellent prices at weekends), you should compare flights for the day you want to travel to all London airports by all the airlines operating on the route - Ryanair, bmi and Aer Lingus.

Anyone pretending they know which will be cheapest on a route like this is merely demonstrating their lack of knowledge.

Try www.skyscanner.net (not spamming, moderators, I find it very useful for playing with all the different options).

Agree with flanner - do a search on Ryanair, air France into city, BMI and aer lingus. Take into account the airport the land at and the cost to get into London and the extra charges and restrictions on Ryanair.

In addition, you may want to benchmark costs and time for airfare + travel to central London against ferry+rail services
http://www.stenaline.ie/ferry/rail-and-sail/holyhead/
Or Eurolines bus services from Dublin to London
www.buseireann.ie

Thank you everyone. Looks like some research is in order.

Factor in the cost of connecting to and from port or airport if you are working on a strict interpretation of "least expensive".