Let's take a careful look at Gibraltar's flag:
Why is this different from any other British overseas' flag? Because it don't have an Union Jack on it. Now, look at the flag of the other fifteen British overseas territories:
Quite boring, isn't it? So today I'll present my proposal for the fifteen! Some of the designs are mine, others aren't (when noted).
My design for Akroitiri and Dhekelia is inspired by the Dhekelia Garrison's flag. For difference, I've added the colors of British Ministry of Defence, to show it's a military base.
My proposal for Anguilla is actually a rip-off of Republic of Anguilla (1967-1969) flag i.e. a banner of current arms.
For Ascension, a banner of arms, too. I added a tiny white fimbriation in the green chevron, just to increase visibility:
For Bermuda, I picked the lion and red of current flag (both representing British influence) but put the lion's face on a triangle, a reference to famous Bermuda's triangle.
My British Antarctic Territory flag proposal is based on coat of arms, too. I put the blue stripes on center and made the white background shine.
For British Indian Ocean Territory, I picked the flag of Chagossians (natives) in exile and put the palm tree and crown from current design. Some minor details is that I put the crown crossing the tree and centered in black stripe, for better aesthetics. This is one of my favorite flags in the series.
Also original is my flag for the British Virgin Islands. The lamp refers to Saint Ursula and the 11,00 martyred handmaidens that give the islands its name. The four stars represent the four main islands: Tortola, Virgin Gorda, Anegada and Jost Van Dyke.
My proposal for Cayman Islands is a simplification of current coat of arms; this design, actually, was reportedly once used by independentist movement.
My flag for Falkland Island is freely inspired in the coat of arms. I removed the ship and shifted the flag for the hoist for aesthetic reasons.
The flag of Montserrat represent, basically, the island of Ireland (due to physical similarities and the origin of many settlers) and the cross. So I made the following design:
Another of my favorite design on the series is that of Pitcairn. It's, basically, a rotated and simplified version of the coat of arms.
For Saint Helena, I kept the symbol bird of the territory, but added a cross in red and yellow in reference to Byzantine empress Saint Helena, that, according to tradition, found the True Cross.
I rotated and simplified the coat of arms of South Georgia and South Sandwich Islands. Now, the two wavy stars represent both James Cook and the two island chains (South Georgia and South Sandwich) that gives the territory its name.
The flag of Tristan da Cunha is just the striking banner of arms:
Finally, my proposal for Turks and Caicos. I couldn't satisfactorily harmonize the three charges of its coat of arms, so I used yellow for sand, green for the flora, pink for the shell and the flamingoes and a lobster as lone charge.
Comments and suggestions are welcome.
Sorry for the delay; I hope the long post is enough sorry.