For those that have had a separate legal ceremony and then a later celebration or blessing, did it make it less special?
I think that really depends on what the B&G* arrange to 'replace' that part of the day. I've been to a fair few weddings where the couple had been legally married before the celebration, and for some of them I really did feel that I had been a witness to their comittment IYSWIM. This meant having someone officiate who knew the couple, a well-structured ceremony that had meaning behind it, heartfelt declarations and (short) appropriate readings. I've also definitely been to ones that didn't feel like a wedding.
Whatever you do, don't:
a) Tell everyone your wedding is taking place abroad, and make everyone feel that, if they don't fly out, they will miss your big day, promising that the actual legal marriage will be 'just a piece of paper'.
b) Decide closer to the time that actually, it will be a bit miserable just to rock up to a registry office in your jeans, so arrange a much more informal day out for everyone who can't make it to the ceremony.
c) Everyone who comes to the legal bit ends up having a really brilliant, relaxed day where you all go to the pub, everyone has a whale of a time, makes fond, off-the-cuff speeches and feels like they've been to a wedding
d) Two weeks later, everyone who's scrimped and saved to fly to your bastard destination wedding (and hence didn't come to the brilliant informal pub day) feels miffed and poor. They;d rather have come to the first ceremony, but here they are, watching you not-actually-get-married for several hundred quid more than everyone else. Later, the B&G muse to each other that "actually, the legal bit was the point where we actually felt married, y'know?"
Can you tell I speak from bitter experience? 
*Or B&B or G&G