Cheapest/best ferry route with destination London?

[placemark123]

Hi all,

Going to London from Belfast at half term... what's the best or cheapest way to go on a ferry?

Previously we go on the overnight Belfast/Liverpool but just so expensive with cabin for the wains plus we are bringing a guest so would need an extra cabin and can't afford it etc.

I guess Dublin Holyhead?

Probably Dublin Holyhead. Are you intending to drive? The ferry/train option to London is really good value if you don't need to bring a car.