You may be right about the Thames tunnel construction not being pressurized.
There are stories of polluted water and gases forcing their way into the tunnel. That suggests that they were working at essentially ground-level atmospheric pressure.
On the other hand they must have been 100 feet below the surface, and that would mean significant hydrostatic pressure to hold off with the sliding-form system (invented by the elder Brunel) without assistance from air pressurization.
Yeah they had a lot of trouble with methane gas getting in and catching fire, but I don’t think they had the knowledge or ability to pressurize the whole tunnel at the time, which probably saved many lives, though it didn’t help the construction time any. The methane got in because apparently the bottom of the river was essentially a bog of sh!T... err, sewage from hundreds of years of the thames being used as a toilet.
Now, Downstream they got the wonderful tidal flushes.