At least one, and presumably several, Southampton scheduled carriers will not be using the extension anytime soon.
The runway extension was “opened” by NOTAM with new runway declared distances, however the procedures for updates to AIP information have not been concluded. If you look at the UK AIP September and October planned updates there’s no mention on the runway extension at Southampton. So no official charts, no surveys of new obstacles based on the new runway surfaces and surrounding areas.
Until official data is available from the likes of Jeppesen, Lido et al, via UK AIP updates, the extension is un-useable. No charts. No performance data using the new extension in either direction.