I've been to London Open House this September. I know it's long ago.
Here are the photo of inside of the Thames Tunnel that was made by Marc Brunel.
I recommend to come early. It was kind of popular and I wait in the ques forever.
It used be an entrance of the tunnel, now that's already filled and it's a huge cylinder shaped space.
The people from Burnel Museum explained how hard and costy to make the tunnel that even didn't work for transporting goods. They run out the money to put the lift in the end.
So, they open as a amusement centre, has a souvenir shop and even dinner party over there.
After the interesting talk inside of the tunnel, people want to get out as soon as they can. Whaaaa!