I tend to automatically want to call officials and clerks "Sir" because I am usually their junior, but instead they are calling me "Sir". When is it appropriate to call someone "Sir" or "Madam" in a UK context?
As Lambie said, you can address them without any title, and it will not seem rude or out of place. If you believe you're younger than them and wish to call them "sir" or "ma'am" out of respect for their age, that's also perfectly fine and polite. If you choose not to do so, that's also just as fine. Bear in mind that respect isn't just through words, but also through tone - so you can speak to them without using "sir" and still come across as perfectly polite and respectful.
Do bear in mind that they call you "sir" for a different reason - that is to respect the fact that you're the recipient of the service they're providing (i.e. in a customer-facing role they're instructed to address the customer with respect, irrespective of the age of the customer).
So, each of you are calling one another "sir" for different reasons which can co-exist and can still be used. Of course, the conversation might get awkward with many "sirs" flying around, so you could limit the "sir" to the initial greeting and final thanks, if you wish.
Don't sweat it, no one is going to be offended by politeness. We could all do with more politeness in this world :-)
Just as one does not properly use "Ma'am" (in the UK) except when addressing the Queen, there is a situation where it seems to be (somewhat) common to use "Sir" in America, but quite abhorrent to do so in the UK: when a son addresses his father.
I am an American who lived for a total of 9 years in the UK, at two different times. I remember television-news segments during the Falkands War which showed British soldiers addressing (then Prime Minister) Thatcher as "Ma'am"; each instance produced shock/horror/wincing among all my (Oxford) friends. And I saw exactly the same reaction several times when an American friend was with his father and addressed him as "Sir."
Since I wrote the above, I checked several (British) dictionaries and style guides, and they all mentioned one thing that suggests my Oxford friends (described above) had less respect for Mrs. Thatcher than for correct English usage. Although indeed Ma'am was the correct way to address Her Majesty the Queen (but only the second time one does so in a conversation; the first time one must use "Your Majesty"), it was also correct to address other women this way, when you are both in the same hierarchical organization (such as the military or the police), and the person being address has higher rank. I don't actually know if the British Prime Minister is also "Commander and Chief" of the Military (as the US President is); but I can imagine that some British soldiers might well think so, and would therefore quite naturally have addressed Mrs. Thatcher as "Ma'am".