I think the two cases that are going on are very separate, the German one is criminal and the London one is civil. Gribowsky's difficulty in mounting any defence in the London case is handicapped by the criminal case in Germany.
His lawyers appear to have taken the safest course and not contested the civil action for fear of jeopardising his German defence, the burden of proof is much less in civil cases and different rules of permissible evidence apply.
There is also the problem of perceived and actual perjury that may be levelled over any evidence he may submit to the British court, if the Germans find him criminally guilty of accepting bribes.
If the German court accepts that their was no bribe but blackmail instead the London court will find it hard to rule differently, despite the judges remarks to the contrary.