Union flag/jack
I'm British and it really pisses me the fuck off when other Brits call our national flag the "Union Flag" instead of the "Union Jack", especially when they then go on to justify their reasoning by saying "it's only a Jack when it's flown at sea" or some other crap which has been cooked up basically for the sole purpose of flinging shit at those who correctly call it the Union Jack whenever they see it.
It has always been the Union Jack ever since King James I united Scotland, England and Ireland under the same banner in a personal union when he came to the throne in 1603, and a quick search on Google or Wikipedia will confirm this.
If you claim to be British and you don't know your British history you aren't worthy of being called a true Brit.