Sadly not. There's nothing between New Cross Gate and Highbury & Islington from the 26th-29th. Through trains run from Saturday 30th.

The main reason is for the building work at Surrey Quays for the new station entrance which is being built next door to Surrey Quays Shopping Centre car park. But it also gives them a reason to do work at stations and on the track which would otherwise involve more weekend closures. If you get on the end carriage back to Forest Hill, you'll see they've closed off the end of the platform with fencing while this work happens.

So Southern will take up the slack during Christmas week on the working days between Christmas and New Year.

The next Highbury & Islington to New Cross Gate closure after Christmas is the weekend of the 3rd-4th February and 24th-25th February and also two weekends in March.