"Andy" wrote

I'd imagine that the potential route would be from Tulse Hill -
Peckham Rye (rather than to Herne Hill and Blackfriars), joining ELL
phase 2 at Peckham Rye.
This would avoid the flat crossing of the South Eastern mainline at
Herne Hill, but at the cost of the Blackfriars to Tulse Hill route.

It makes sense for the Wimbledon/Sutton Loop service to become
self-contained from Blackfriars. It will keep to the west side between
loughborough junction and the new bay platforms at Blackfriars, so will not
conflict with Thameslink at all. It also minimises conflicts at Tulse Hill.
Currently off-peak from London Bridge via the South London Line there are
2tph to Victoria via Denmark Hill. These will be diverted to start from
Bellingham.
2tph to Smitham via Crystal Palace
2tph to Beckenham Junction
2tph to West Croydon via Selhurst. These do conflict with Thameslink trains
between Tulse Hill and Streatham Junctions.
While Blackfriars may not be the ideal London terminus for all Wimbledon
Loop passengers, they do have a wide choice by one change, at Wimbledon to
Waterloo or LUL District Line, at Sutton or Herne Hill to Victoria, at Tulse
Hill to London Bridge, at Elephant to LUL Northern or Bakerloo Line (though
this interchange desperately needs improvement), and at Blackfriars to LUL
Circle Line and to Thameslink.

Peter