Bob
To be serious for a moment one reason why you can’t fly the union flag is that it’s illigal (daft but true) for civillians to fly the union flag at sea and a private vessel should not really fly any of the traditional english ensigns.
The white ensign, a white flag bearing the cross of St George and with the upper corner near the staff occupied by the union device, is the exclusive flag of the Royal Navy and the Royal Yacht Squadron.
The red ensign is a red flag with the union device in the upper quarter near the staff, and is flown by British merchant vessels.
The blue ensign is a plain blue flag with the union device in the upper quarter next to the staff, and is flown by the Royal Naval Reserve and by certain yacht clubs. The union flag and the blue ensign are also used with various additions to denote various officials or departments.
It is traditional for English naval ships to fly the Union flag from the jack staff on the bow, which some believe is why it was called the Union jack, and the flag is only refered to as the Union Jack when it is used at sea on land it is called the Union flag.
I hope that this will go some way to explaining the protocol regarding the positioning of the Union flag.
Paul