Is there any verification that the 25 FPS is correct?. Stop motion is generally shot at 12 FPS, since it’s too much work to animate any faster, and then doubled to film speed. That’s how they made Curse of the Were-Rabbit. The only reason for them to be 25 FPS would be if they sped up the video and produced the audio at 25 FPS for PAL broadcast purposes
Pitch is really only half of what you need to figure it out as well, you also need to check the timings. Pitch is easily fixed, by adjusting the tempo, which keeps the same pitch, but changes the length (most likely how they fixed it on the VHS).
The best way to figure out the proper framerate is to compare to the soundtrack/score, if it exists officially anywhere. You would check the pitch and length against the video audio. One might be the same pitch, but longer or shorter
Alternatively, surely someone at Aardman might know. It might be worth reaching out via e-mail or social media
[Emphasis mine] I think that’s exactly what was done. It was shot in twos, and was presumably sped up to 25fps for audio sync. I’m willing to trust the UK broadcasts of The Wrong Trousers over video releases with the wrong pitch, that happen to have the exact same pitch difference you get from a PAL-NTSC slow-down.