Where in Berlin are they going to find a spare platform they can install UK immigration control infrastructure around (of the sort they have in Paris/Brussels)? Given the long distance such a journey would cover, the demand would be lower than from a 3-hour Eurostar trip, meaning that taking a station platform and enough space for passport control booths just for a few passengers would be of dubious value.
Where in Berlin are they going to find a spare platform they can install UK immigration control infrastructure around (of the sort they have in Paris/Brussels)? Given the long distance such a journey would cover, the demand would be lower than from a 3-hour Eurostar trip, meaning that taking a station platform and enough space for passport control booths just for a few passengers would be of dubious value.