Hi Dayvid,
I also had the idea of using a displacement map as mentioned by Trevor because I wanted to preserve the shading of the building as to add to realism. I did not have much success with the process.
I did use a variation of what has already been mentioned.
I first found a larger file version of the Union Jack, yours was way too small. I converted it to a Smart object and then used transform>distort (some warp) to fit it to the end of the building.
I lowered the opacity of the now transformed Union Jack (so I could see through it) and added a layer mask.
With the layer mask selected and I used Tom Mann's very useful tip in isolating the windows and, of course, filling the selection with black (option + delete).
I wanted the UJ to be at about 75% opacity. This is what it looked like.
[ATTACH]37578[/ATTACH]
Not very realistic.
So it would be visible on the UJ, I created a new layer(s) above the rest and used the Polygonal Lasso Tool to select areas of the buildings face that were darker. After I made the selection, I filled it with black and reduced it's opacity. I show these here on a white background so you can get the idea.
[ATTACH]37579[/ATTACH]
I also added a new layer with a gradient designed to mimic the gradient on the building. I outlined the end of the building with the Polygonal Lasso Tool and then used the Gradient Tool, set to foreground to transparent with black being the foreground color, to run a gradient within the selection. Command + D to deselect. Remember, these are going on top of the UJ layer. Again, this is on a white background layer that would not be there when you do it!
[ATTACH]37580[/ATTACH]
In the end (after removing the white demonstration layer) you get a fairly convincing composite.
[ATTACH]37581[/ATTACH]
It could use a lot more tweaking but you get the idea.
