It depends on the scope of potential celebrations that could be planned for this event.
On a small scale its likely to be a cake and a blog post/local or Railadvent article at one of the stations along the line.
On a larger scale, perhaps something decent could be made of it, maybe repainting Robertsbridge National Rail Station in BR Southern Region Green? Perhaps the Rother Valley Railway could have a shuttle train with an SR V class locomotive, as they did for the KESR's 50th anniversary with Britannia. If so, it would be a nice last outing for Repton, provided such an anniversary isn't after its boiler ticket expires, which is also happening in 2027.
Hopefully, some railtours could come of it? It would be nice to see Tangmere come back down South for a railtour, especially for the Hastings line. If it were running back on the mainline by next year, I would like to see N15 'Sir Lamiel' run the Hastings line on a railtour.