Let us render this into more intelligible modern prose, and see if that helps.
No matter [what] the season [may be (in other words, "regardless of the season")], these combined features [in other words, "these geographical features, taken all at once,"] present to the climber [in other words, "present the climber with"] a uniquely [in my opinion, this word is redundant with the phrase "nowhere else" appearing at the end of the sentence] varied and demanding challenge [that may be] found nowhere else [in other words, "that is not to be found anywhere else"] in Britain.
So, as revised into our modern (and far less poetic) language, the sentence might read: "Regardless of the season, these geographical features, taken all at once, present the climber with a varied and demanding challenge that is not to be found anywhere else in Britain."
It might also be pointed out that the phrase that you seek to identify may be placed at the end of the sentence with no damage to meaning, and, after revision into our clanking contemporary speech, such placement lends it a great deal more emphasis:
"These geographical features, taken all at once, present the climber with a varied and demanding challenge that is not to be found anywhere else in Britain, regardless of the season."
You should now easily be able to see that this is a dependent clause (or "subordinate clause," depending on your local pedagogical tradition), set off by a comma. I hope this helps. --DKR