Thanks for your advice! It's not clear to us either whether we have to go to the closest designated office. I'd like to take your suggestion to email UKBA to ask, but I can't find an email address to do so - I looked online and found the contact information for the Croydon center, but did not see an email address listed. Thanks in advance for any help -
I was suggesting contact the register office. We emailed the register office in our council (which happens to be a designated one) to get basic info (in writing for my Fiance Visa) and later called them up to ask a few questions and book to give notice. Depending on who picked up the phone, we got different answers to some questions..... email might be better so it's in writing and they have time to ask the head registrar and get back to you (hopefully with the right answer). Maybe send an email to a few of the designated offices and hope their answers are the same!
Edited to add-
However you do it, get the ball rolling sooner rather than later. If you or your partner is here on a Fiance Visa, you've only got 6 months to get everything sorted. Once we got my name on a bill (which we delayed a bit and then it took a while to actually go through) to prove my postcode, they were booked out a month to give notice and a further month for a ceremony. At the end of the day, we got it all done within about 4.5 months which wasn't cutting it too close, but I'm glad we didn't wait longer to start to process or have any major hiccups along the way!
Edited again to add-
I just saw you're here on a Tier 2 visa so the Fiance Visa stuff doesn't apply, but if your visa is ended soon, you've got that deadline to consider.
If not, then take your time!