Even though these are based on the official Poirot canon, its not unusual for a long running tv series to feature one or more stories set in a film and/or tv studio. Clearly, as the (first) series was shot at the historic Twickenham Film Studios, that existed during the 1930s anyway, it makes perfect sense to shoot this episode there, as unlike some certain other studio facilities available in the UK, that either never existed in tbe 1930s, or now have changed beyond its 1930s period style, there's still enough of the original site to use, even with additional (digital) Matte Paintings. It also works as a bonus playful "behind the scenes" story, showing us where Poirot was filmed in 1989. The use of the Hoover (vacuum cleaner) factory as the exterior and some interiors, of the film studio complex, instead of just Twickenham Studios 3 soundstages, also implies that Parade is a gigantic production facility, not unlike MGM.