I've never gone Dublin to London using the ferry and train, and God willing I never will!

You should be able to get much cheaper air fares than the BA flight you quoted. Try skyscanner.net.
There are five london airports with connections to Dublin....Heathrow, Gatwick,London City, Luton and Stanstead. The last two are further out from Central London but have ok train links. Skyscanner is showing a one way ryanair flight from Luton to Dublin for around €30 for the end of July.

Hope that helps.