Pan Am earlier in the 1980s did a LGW-Houston flight, with an L1011-500, which the Bermuda Agreement did not allow at Heathrow as it was outside the geographical US boundary. Later, when they sold their Heathrow operation to United in 1990, they continued to run Miami and Detroit flights from Gatwick with the A310, as only two US carriers were allowed at Heathrow.
The Miami flights worked through a notable range of carriers around 1990, almost a different one every year, several US carriers had a shot at it - United, Continental, and others.
The German flights didn't operate nonstop to Berlin but went through other German points. They were part of the sale of Pan Am London routes to United, separate from the rest of Europe which went to Delta. United positioned a few 727s over to Europe to run these and other European connecting flights for a while, which all came together each late morning at Heathrow where the 727s parked on the remote stands between T3 and the northern runway.