According to WIKIPEDIA, there was no performance of The Planets until 1918 and no public performance until 1920, so it's quite possible that the "Jupiter Hymn" - a piece which exists in isolation from the rest of the Jupiter movement - was interpolated at a later stage. Parry actually died in 1916 so if he didn't a) personally know Holst and had access to his work, or b) employed spies it would obviously have been impossible for him steal the tune from Holst as he wasn't alive when it was (supposedly) performed. It's not entirely impossible that it was a coincidence but this is highly improbable because both pieces are so very similar.

EDIT: I was in error. Parry died in 1918, not 1916. But, Jerusalem is still down as 1916 so SOMEONE was a thieving little magpie and my guess is that it was not Parry. However, my initial intention was to ask: isn't it a little strange that the similarity between these pieces of music is never remarked upon (not even when they are played at the same occasions/events), yet a lot of people have said that Braveheart is similar to "Jupiter"?