Transport may not be much better on the 26th. The past two years, the Hex was shut down due to planned maintenance on the rail line into Paddington. The Piccadilly Line was running in to town, but was packed. The District and Circle Lines were mostly shut down due to planned maintenance for most of the week between Christmas and New Year - check this calendar https://tfl.gov.uk/forms/12392.aspx a bit closer to your departure to see if any shutdowns are scheduled.