As
@Dogtrousers says, Britrail Passes are not valid on the underground, which seems to defeat the object: why offer a railpass for the country if it doesn't include the main transport network of the capital city?
As a Londoner I found that quite normal. Trains and tubes (and buses) are entirely different animals. Bu I can see how someone not living in that environment might find it surprising. But what might catch you out is the Overground - this looks like an ordinay train, is an ordinary train, but is run by Transport for London so probably
wouldn't be included. Whereas an ordinary commuter train (in some cases running on the same line!)
would be included. It won't be valid on the Manchester, Nottingham or Croydon trams either (to name three tram networks that come to mind)
Anyway, if you have a contactless credit or debit card, you can use these on the tube barriers so it's quite easy (if not free!)
Confusion works the other way: I went to Berlin by train recently. We spent ages wandering round the Hauptbahnhof in Berlin trying to find a machine to buy a ticket for the S-Bahn to our destination (there are very few in the main station itself, as they are actually on the S-Bahn platforms). Faffed around, got the tickets, got to the hotel and read the small print that said that S and U Bahn were included in our main train ticket. Doh!