I loved this film when I was 5 years old. They took the school on a trip to the movies. Very English. I liked the sound effects. I used to dream of having a super T-shirt. I managed to find the full movie on You Tube. Still worth the watch 35 years later.
Sammy's Tiger T-shirt made its way into a factory where a man was carrying out experiments on textiles to make them indestructible. Sammy and his friend sneak into the lab and try to get the shirt back. They accidentally set off the machine. The shirt gets zapped by radiation that gives it superpowers. Sammy and his friend escape but the men from the factory chase after them and try to get the shirt back.