And also https://www.derelictplaces.co.uk/main/forum.php and https://www.28dayslater.co.uk for the more adventurous, both of which have underground subforums.

I should mention that trespass on live railways (and a few other places) is a criminal offence throughout the UK, so you must get permission if you want to go anywhere on them not open to the public. Subbrit occasionally arrange tours of closed tube stations such as Down Street and South Kentish Town.

Both sites have intriguing reports of closed/abandoned Tube stations, tunnels and bomb shelters. Very impressive.