[null and void]

I’ve heard it called a British flag and a Union Jack, but never a British Jack nor a Union flag...

[Hawker Tempest]

Technically the Union or the Union Jack is acceptable. A jack flag is generally termed as the colours of a ship. So ....jack flag.

[the scotsman]

The Union Flag is in fact its correct name, although it is very rarely used.

[the scotsman]

The Union Flag is in fact its correct name, although it is very rarely used.