[TOMT] Movie scene where kid doesn’t want to jump into a swimming pool

I cannot remember the name of the film or possibly a TV show. There’s a flashback scene and a little boy is scared to jump in the pool but his dad is there and keeps promising to catch him. The little boy eventually does. He goes under the water at first but the the dad catches him and pulls him above the water. The little boy is all happy that he did it and says something like ‘i did it!’. I seem to remember them being British and there may have been an older brother there too? Just have this image of the dad standing in the pool with the little boy on the side and he keeps saying I’ll catch you