But how do you say it in British English?

As I've said before, the students are learning British English and I am learning it right along with them. True that I have a slight advantage because I know American English, but I still learn new words/phrases every day. It's sort of a testament to the fact that the two are different. And the students want to make sure they are learning British English because that is what they will be tested on. If they say something the American way, like: 'At the university you can study Urban Geography,' they would be wrong. They have to say 'At university...' (without the 'the'). They also say 'at hospital' and 'at weekends'. There are differences in verb conjugations too. Learned and learnt, for example.

One day in conversation class I felt like I was being set up. A student was talking and asked how to say 'cola' in English.
"Line," I said.
"Queue," said another student.

Then the speaking student asked how to say 'galleta' in English.
"Cookie," I said.
"Biscuit," said the other student.

I thought the third time would be the charm, but "¿'Farmacia' en inglés?"asked the speaking student.
"Pharmacy or drug store," I said.
The other student shook his head, "Chemist," he said.

So you can see how easily I can lose credibility in a short amount of time and why students are skeptical that the answers I give them are correct.

But there have been a few 'in your face' moments too (I say it all in good fun, of course). The other day students from the second year class were talking about their families. One student asked how many brothers (hermanos) her partner had. 'Hermanos' can mean either 'brothers' or 'siblings' in Spanish so I thought it would be a good opportunity to teach them the word 'siblings'.

"Siblings?" one student asked, "is that British or American?"
"Both," I said.
"But what do they say in the UK?"
"Siblings," I said.
She reluctantly wrote it down. I could see her making a mental note to look it up in her British dictionary when she got home though.

My favorite moment when I was right was in a first year class. The students were going a role-play activity wherein one person was a store clerk and the other was a customer. The script read, 'Hello, how can I help you, Sir.' As you might imagine there are some females in the class too, so they wanted to know how to say the feminine form of 'Sir'.
"Ma'am," I said.
"But what do they say in Britian?" asked the student.
I considered for a moment if there was a more British alternative.
"They say 'Ma'am'," I said.
"Well I'm going to ask José (the teacher) when he gets back." (He was downstairs making copies at the time.)
When José came back the student waved him over to her desk. By this time I was helping another student, but I had one ear on the conversation.
"Ma'am," José said.
"Yesssssssssss," I said to myself.
"I just had to be sure," the student told me. "I'm sorry, I didn't mean to be rude."
"It's ok," I replied. "I understand."
And believe me, I do. I don't want to learn the South American word in Spanish. I want to know the Spain Spanish word.