Does an illuminated speed restriction sing on LUL indicate that a train
is entering a speed controlled area?

The digits on the speed restriction are white on a black background, I
notice.

I have seen such cases on the District Line's eastbound track between
Monument and Tower Hill, just before the points leading into the bay
track at the latter, and just by the starter on WB2 at Gloucester Road.

Are there many such speed controlled signals on LUL? I'm not talking
about draw up signals, such as those we might see at Baker Street,
Kennington or eastbound at Whitechapel.

I also wonder what the difference is between speed-controlled signals
and another one that I saw just by the portal at Golders Green. In this
case, the signal simply had a sign next to it that said TS15, indicating
that it was a timed signal and to approach it at that speed.