I can't make any suggestions about London as it is years since I have been there and my LU knowledge is very sketchy, but I really enjoyed what I saw.
Could there be scope for something like "Secrets of Britain's Railways"?
There must be many abandoned bits which are not accessible.
For instance where I live on Merseyside we have the Merseyrail system which is underground in the Birkenhead and Liverpool area. T
here is definitely an abandoned pedestrian subway from Shore Road, Birkenhead built to provide access to the docks down to Hamilton Square Station - normal access is via lifts. (Similar subway on Liverpool side exists but is still in use).
After watching a couple of episodes SOTLU I started thinking about the old Mersey Railway River Cabin signal box which was located under the River Mersey. There is a contemporary engraving which has been published a few times.
The box was abandoned when the Mersey Railway was converted from steam to electric and the signalling system automated in 1903.
Out of curiosity I posted a message on a local railway Facebook page and someone who works for Merseyrail had been down there and was able to provide a couple of phone photos the brackets which supported the box.
There must be many other nooks and crannies above and below ground all around the UK worth exploring. I must say I am hungry for more and I dare say many others are too!
John
Another bit that I find interesting is when the 'link' line (Merseyrail Northern Line) was being constructed in the 1970s a lot of the actual tunnel formation south of Liverpool Central had to be reworked/lowered etc & in doing so the contractors as they lowered the trackbed actually intercepted & indeed emerged right through & into the top of the disused Edge Hill to Park Lane (Wapping) tunnel.
This was not by accident but part of the plan, as there were further future long term aspirations to eventually also link up to & use a large part of the Wapping tunnel to provide a link from the Merseyrail Northern Line to the Merseyrail 'City' Line at Edge hill, & as such Liverpool Central South Junction was configured in such a way as to provide a future 'burrowing' junction so that it could eventually be done if & when funds became available.
At Liverpool Central South Junction, the initial headings were started to facilitate this & these concrete lined headings are still there to this day.
There were also other & more different plans to build a totally new tunnel under the city to provide a new station to serve the university area of the city, & this tunnel if built would have not used the Wapping tunnel but instead broke into & linked up with the Waterloo tunnel instead to link up with the Merseyrail City Line at Edge Hill.
In this scheme to use the Waterloo tunnel & university link, the Wapping tunnel would have not been used to carry track, but instead would have been used as a draught relief/ventilation shaft & emergency access.
Urban explorers who have ventured into the disused Wapping tunnel have come across the point (just to the west of Great George Street) where the 1970s Merseyrail Northern Line intercepts the disused Wapping tunnel & on their photos (& videos) it can be clearly seen how the arched crown of the tunnel roof suddenly changes in height & shape dramatically, as the tunnel roof suddenly becomes much lower because it is now crossed by what is in effect a modern double track reenforced concrete bridge at a height of about 12ft up from the Wapping tunnel trackbed.
Although all secured, totally bricked up & sealed from the disused Wapping tunnel it is possible to hear the trains pass by above on the active Merseyrail Northern Line.
youtube.com/watch?v=ZRGDoMIgicg&t=1266s
liverpoolecho.co.uk/news/liverpool-news/disused-rail-tunnel-could-reopen-28331473
An old timer on BR once told me that there's at least 4 to 5 miles of disused railway tunnels beneath Liverpool & Birkenhead, I haven't bothered to work it yard by yard but I reckon that he was very likely right.
Wapping tunnel.
Victoria/Waterloo tunnel
Dingle tunnel (LOR) (now partially collapsed)
Walton On The Hill tunnels
Kirkdale No1 tunnel (fast line bore)
Kirkdale No2 tunnel (fast line bore)
Canada Dock tunnel
Monks Ferry tunnel (Birkenhead)