My wife and I are spending a few days in London before flying from Heathrow to Frankfurt and catching a train to Göttingen from the station at the airport. Do we have passport and customs checks now with all the Brexit complications?
Yes. You would have gone through passport control pre-Brexit because the UK has never been party to the Schengen Agreement. Now you will go through both passport control and customs.
The UK has never been a part of the Schengen zone, so it has always been necessary to go through immigration when entering Germany. Brexit is irrelevant. Customs is just a walk through.
There is no connection between Brexit and immigration and the Schengen zone. Two completely different issues and objectives.