We've taken the Stena line high speed ferry from Stranraer in west Scotland to Belfast in Northern Ireland twice in the last two trips. It's fast, huge, and a lot of fun. If you catch the rails right, it's about 8 hours from London to Stranraer. First trip we missed a connection, and spent the night in Carlisle, which turned out to be an unexpected treat, as we were pretty worn out from travelling. Do a search for rail/sail or belfast to london train to find out additional details.
My $.02 (whats that with the current exchange?)
Glenn in Tucson, AZ 2 months till the next trip to the UK!