Not quite true I’m afraid, maximum speed on LU is 60 mph, although there’s not too many tunnel sections where that’s achieved. From memory the Jubilee is the only one, although the Northern and Victoria reach 50 mph.
The Met Line can reach 60 mph in some open sections, although that’s with good old signals (for the time being).
The Northern might be next to join the 60 mph club, technically the Northern is already 60 mph on paper, but with a speed cap applied at 50 mph as the paperwork for 60 mph has yet to be signed off, having run into issues. Not sure where things are with that at present.
It’s also worth a mention that the Central Line setup supports 60 mph (actually 100 kph) and has done for roughly two decades, but the trains are restricted to 85 kph due to their mechanical issues. These high speeds are only in open sections however, I don’t think there’s any tunnel sections on the Central which go above 65 kph.
As a final footnote, 60 mph wasn’t uncommon in the past. Indeed many years ago it was common practice to try to get the speedo needle “off the clock” on certain fast sections like High Barnet to Totteridge or Oakwood to Southgate, which would equate to 60 mph or more. This belies the point I made elsewhere that things are safer today, although as with many things in life it depends how much risk you’re/we’re prepared to tolerate.