Find Your Cruise
The northern lights are usually by specialists companies as its later on in the year and usually on smaller ships. Do a google search for northern lights cruises and you will come up with info, then ask cruise.co.uk if they can better the prices when you find what you are looking for.
The only cruiseline I know to offer this is Hurtigruten aka Norwegian Coastal Voyage. They operate a range of winter trips all under the Northern Lights.