Having used the tube for several years, I've never yet encountered a
revenue ticket inspection on board a tube train or within station
passages [1]. Do LUL ever bother checking tickets randomly on the
network or do they let the ticket gates do the job?

[1] I did get my Oyster card swiped by someone on board a DLR train to
Custom House last Fri. If I hadn't touched in my annual Oystercard,
would I have been liable for a penalty fare? The reason I ask, is
that the station tannoy always reminds PAYG customers to always touch
in and touch out, but it doesn't mention annual Oysters.