I see Tim is working on a new series to be shown later in the year on
Yesterday. From a tweet this morning of his:

One of the few +ves of commuting on the tube (and filming a TV show called
#SecretsOfTheLondonUnderground) during a lockdown is that recently I’ve
been walking around London’s underground network a lot, and at times,
almost *entirely* by myself.

That led me to an announcement from two weeks ago:
https://realscreen.com/2021/03/16/uktv-commissions-secrets-of-the-london-underground-from-brown-bob/