The M25 is a motorway (70mph highway/Interstate) which circles around London. It's illegal to ride bikes on any motorway, and there are no bike lanes on motorways, nor should there be. And I say that as a cyclist.
In this case, there is no good reason a cyclist should be on the M25 - there's almost definitely a quicker and calmer way to get to where you need to, without bypassing around London.
They intentionally removed this feature years ago. It was possible to reenable via a dconf setting for a while but I believe that was also eventually removed.
So annoying.