Pretty great collection of easy to access walls from Holborn.
Mile End Wall has an ace little training room that apparently you can even
preview on google maps. Always used to be quiet in there. Under half an hour on the central line from Holborn - worth noting they close at 21:30 on weekdays.
Stronghold also has a great board and good campus board set up. Take the Picadilly Line from Holborn to Finsbury Park, walk across the platform and take the Victoria Line a couple more stops (google suggests changing at Kings X, but that's much more faff) - again around half an hour.
Yonder, same as for Stronghold but another couple of stops on the Victoria Line. Symmetrical board with all wooden holds, Moonboard and under used campus board.
Westway or the Castle also decent options, and have the best roped climbing.