Thanks, frank1e74, for alerting me to the issue of access to Holborn station -- I googled and found out the details. This is very disappointing to me, as I am arriving next week and was counting on using the tube to go places. Do I understand correctly that the Holborn station entrance is closed 7:30am and 10:00am daily, and there is currently no changeover from the Central line to the Piccadilly line at the station during the same hours ? Bummer!
Get an answer from Citadines Holborn-Covent Garden London staff and past guests.