I can’t tell you for sure however, the HHOF is excellent at keeping track of such things. It doesn’t make too much sense that the longest series would be in the UK, but given the game has its origins in Canada it is more than likely that the game would have made its way back to the British Isles.
Also, if you can find a book called Total Hockey it should be able to confirm the information, but since it is esentially a NHL publication, the source should be the same.
Maybe a TO doper could be bribed to go to the HHOF and confirm the answer?
If you do ever get to Toronto, the HHOF is worth a visit.
McGill University in Montreal claims to be the birthplace of “the game we call hockey today” in 1873. Their club was formed in 1877. The Cambridge University Ice Hockey Club was formed in 1885, followed by shortly by the Oxford University Ice Hockey Club the same year. (The Oxford site says that there is “some evidence that the club may have existed as early as 1879.”) From the Oxford site regarding the Cambridge-Oxford rivalry:
I can easily believe that this is the oldest ongoing rivalry.