JAL has bases in LHR and FRA. The LHR based crews also position all over europe to operate from european cities to Tokyo and Osaka. There are typically two to four LHR based crew on each flight.
I guess the difference between the LHR JL crews and QF crews is that JAL does not have the base as a cost saving measure. The LHR based crews are on-board for their (english/french/german) language skills as the japanese crews speak very basic english. They are also there to interact with the european passengers on JAL flights. The same as BA with our NRT based crew. Not to save money but just for the language/culture issue.