Hey, my fiancé lives in Northern Ireland, and unfortunately you HAVE to fly to London- it's a total pain! Here's a link that will help with details about the medical. Although my link is for the K-1 and you are doing a IR-1/CR-1, I don't see why the address would change... I wouldn't trust the local office that says they do visa medicals. It sounds really convenient though!
If you need immunizations for the IR-1/CR-1, try and get them from your doctor in N.I, because London will charge A LOT for each shot. Maybe you already know that, but saving some $ during this process makes a huge difference so I figure I'll share just incase. If you do not need vaccinations, your arms are lucky! Hope this helps!