Per this tweet: https://twitter.com/thetransporthub/status/1540659424892682240?s=21&t=R2GdYvF5naKFvVOjRN5xGgSouthern? Posting elsewhere suggests they go to Hornsey first. I note the conversation above and post from 'Class 466' indicates that it isn't necessarily the six 387/2s on the GN that go south and does corroborate the Southern idea. Ultimately the point appears to be that Southern gets six 387s of some sort that can help with stock pressures.
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Still no detail as to what/where they will be used, but I would assume Brighton fasts to internally cascade 377s.