There will certainly be records of ferry vans trapped in the UK during WWII in archives of the REC kept at Kew. The main reason for this is that while away from their home network a charge was made on the current user. This was record of fees due to various continental railways was kept until the wagons returned to their owners. In normal times this would be at most a week or two.
There were 696 wagons trapped by the fall of France, Belgium and Holland, mostly French wagons, many were used in the UK during the war.
A total of 547 ferry vans were loaned to the WD during the war, who converted them to various uses. For example they were used in tank transportation trains to carry stores related to the ramp wagons. The length of the wagons, up to 42 feet, made them preferable for conversion, in particular the vans from Germany and Czechoslovakia.
An example of the paper trail on these wagons, that exists at Kew, is the case of a van that was destroyed by fire while on loan to the WD. Significant paper work was generated between the parties concerned as to the value of the remains, the compensation due, and who was going to pay.
What is remarkable about this incident is that wagon in question was German owned. The fact that the allied bombers were trying, day and night, to destroy as many similar vehicles was of no consequence to the pen pushers, the paper work had to be in order for when peace finally broke out.