May I suggest your son find a storage unit in Germany close by, then he can hire someone to move them to storage.
That way they are no longer the responsibility of the ex-girlfriend or her family. He should be able to do this with the help of the internet, allowing you to step away from the situation.
He can then deal with the items whenever he gets a chance to return. There may be some issues moving personal goods from one country to another, especially now with Uk out of EU., and of course Covid.
Estranged Son and Future Granddaughter
To think that London, or anywhere else for that matter, does not belong to any one demographic