Answering my own question since no one here had the answer! I called Cunard today.

You depart the ship in 'touring' clothes. Your luggage is removed per normal and forwarded onto the train to meet you in London.

Passengers are sugest to take a garment bag containing their dinner clothes when departing the ship. These will be stored on the tour coach until the transfer to the train. The train lounge are has applicable changing rooms for guests to change from touring clothes to dinner clothes.