This headquarters can be quite difficult as there are quite a lot of the more serious enemy types and a lot of snipers in here and they can kill you quite quickly.
I recommend approaching the restricted area from the viewpoint directly north. Climb the storage area here and use it to access a climbing route up to the top of the large building with the large chimney. Once up top, clear out any enemies on the rooftop before heading to the southernmost corner of the rooftop. You’ll see the flag we need to cut down below – we’ll get that in a minute! Behind this, inside the wooden structure, you will also find the British hostages that we need to free.
Step out onto the beam jutting out from the corner (it’s overlooking). Turn on eagle vision and look down to the right, the gang leader will cut laps of the area below, so wait here for a short time and wait for him to appear. When he does, perform an air assassination on him. If you are lucky, no one will have noticed, however it is more than likely that you’ll have to fight 1-2 enemies at this point. Either way, quickly run over to the blue objective marker inside the shed to kill them out of the sight of other enemies and free the British hostages when you are done.
Now make your way back outside, climb the structure in front and interact with the flag (there is a haystack next to it if you need to become anonymous) to complete the takeover.