Plagiarism is a pretty strong term to use. There actually aren't all that many successful plagiarism lawsuits when it comes to TV and movies, mainly because you can't really claim most of it as unique. For example, a lot of model train fans wear a hat while they play with their set; it's something that goes back decades and can be seen in advertising from before it was used in LOTSW. As for this particular case, each of the following it possible:
A. The writers of that specific Simpson's episode intentionally copied the scene.
B. The writers had seen the LOTSW episode but did not with intent create a similar scene.
C. The writers did it as a way of honoring LOTSW.
Either way, I doubt Roy Clarke or the BBC care.