Actually, "granted that" is a conjunction which means "even assuming that", and "granted" means "admittedly; it is true". The two are different. Facet means aspect. The question arises as to whether the writer of the sentence (it's not a complete sentence!) wrote correctly. Then the question arises as to whether the other responses are correct.

Granted here means "I agree".

In this context 'granted' means 'because of something', or 'admittedly'. In terms of your sentence this is referring to 'not joining the single currency'. 'Facet' means a part of something. e.g. Admittedly, by not joining the single currency we will preserve one part of our national identity.