Only 2 possible identities for the so-called 'terrorists'

I don't know. To me, given the frightening evolution of communautarism in the western societies, the only believable conflict would happen between nationalists and another group belonging to one of the "new" communities ; and the only people capable of doing that would be the Muslims, indeed (because they have the number, a strong model of society based on their religion, etc.). So, I'd tend to go for n°1 too. BUT, before such a thing happen, England would have to go through two or three decades of societal and civil degradation which would make it look like the Balkans... and it didn't feel like that to me at the beginning (although we don't see much, I know). Plus, the conflict would certainly not resolve itself as easily as at the end of the movie, of course...

Political correctness might be the reason why McDonald wasn't more explicit about the identity of the enemies... (after all, England is the best at this game) But I don't think so. I think he didn't bother because it just wasn't what his movie was about.

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"This is ridiculous. If I'm going to die, I want to die in Manhattan."

To specify the nature of the terrorists in this movie would have given it a political slant which it is better off without. As can be seen from these message boards, every Tom, Dick and Harriet wants to give us the benefit of their political opinions and I am happy that the makers of this movie did not feel the urge to do the same.

The men in the woods felt irish to me... but they could also just be some roaming kinda-local raiders

"It's beer o'clock. Where's my riot?"

Actually, there was no indication whatsoever that they might be Muslim, and I believe this is intentional, the author did not want speculation along these lines, as it is completely irrelevant to the storyline.

It is not easy to divide people along any line you want. Take as an excellent example the fairly recent war in Bosnia. In the media, it was presented as Christians and Muslims. However, Serb soldiers, who are nominally Christian, were actually bombing away all over Muslims and Christians alike in the city of Sarajevo, they hated both equally. How so? Simple, it was a rural versus urban conflict, where the rural Serbs felt excluded from the more cosmopolitan urban dwellers, which is why they were showering them with bombs in revenge. And even more so, the motive was pillage, rape and personal ambition. So, you could divide them along any line that pleases your argument:
- Christian vs Muslim
- Serb vs. Croat and Bosniak
- Rural vs Urban
- Pirates vs Townspeople

It is exactly the same in Syria today. The conflict can be viewed as:
- Sunni Muslim vs. *beep* Muslim
- Muslim against everyone else
- Alawite against everyone else
- Fundamental Islam vs. moderate Islam
- Pro-American vs. pro-Russian
- Pro-Iran vs. pro-Saudi
- Civilized vs. Middle Ages mentality
etc.

Fundamentally, all these are excuses, the real motives are power, pillage and rape. Just look at ISIS, they make it 100% transparent.

In this particular case, it could be English indigenous (although what that is, is really open to interpretation) against immigrants of all sorts or rural vs. urban or Microsoft fans vs Apple fans - whatever pleases you, the movie very tried hard not to reveal, as it would spoil the story.

Two more options:

3) The IRA. The Irish republicans are still armed and active.

4) The USA. The US is not Britain's oldest ally, as described in London has Fallen (actually the other way around). The USA was an enemy in the 19th century, was planning war against the UK as late as the 1930's, and most American people favoured Nazi Germany over the UK in the late 1930's.

SCOUSERS!!! I thought the guys in the woods were Liverpudlians (I knew they'd rise up one day). Was there any indication that they were terrorists or just unattached lowlife taking advantage of the emergency?

I think the movie was not being explicit because it was trying to make it all about the teen romance, but as a result the movie fails. With some thought and ingenuity you can make a bottom up view of a conflict that is still comprehensible.

My take on it is that there were two different factions both of whom were rather unpleasant.

The setting was not particularly futuristic so the only people likely to have the numbers and organisation needed to take on the government in 25 or so years time would be Muslims. Still a relatively small share of the population by then, but far higher a share of the men of "military age"

This would be faction 1, the shadowy figures usually dressed in black. It is they I believe who were raping the women in that night scene and were engaging the armed forces.

Faction 2 would seem to be some sort of far right response, incorporating most of the conventional army, who seemed to be both corralling the civilian population and carrying out massacres of both ethnic minorities and whites who would not follow the new order (hence the bodies she discovered in the farm yard)

The likely end point was some sort of government of national unity, like in Bosnia, that called a ceasefire. Hence the "new" government.

But you have to infer all this and even someone as self-absorbed as the lead character would have found a bit more out about what was going on.