Hello Hector
There is a great deal of confusion regarding the White Ensign but some time ago I had to research the subject for an M1 project.
To the best of my knowledge all Royal Navy ships and submarines fly the White Ensign from the Jackstaff when underway on the surface. The flag can be carried on a gaff or flown from the starboard yardarm when at sea.
When moored or alongside the White Ensign is flown from the stern and the Union Flag is flown from the bow.
I hope this helps but I strongly suggest that you carry out your own research to confirm my comments.
Paul
Usual disclaimers apply 