Your data are outdated (forgive my pun). Russia claims that its bombers have destroyed 500 military infrastructure objects of ISIS. If we consider that half of the 30k ISIS fighters (15k) are in Syria and that each of the objects was guarded by at least a platoon (30 men), then 500*30= 15 000. Syria is free from ISIS (according to the Kremlin).
I'm not sure they must do it. They can cross the Channel from elsewhere as if on business of their own (can you figure out the preachers among them?), then come to the mosque and start their preaching. Alternatively, they can preach at secret meetings without any license. And, finally, who told you that they are all FOREIGNERS? I'm sure by now there are some UK citizens who can do this (consider France and Belgium as an example).
I'm surprised to hear such a discussion from people who have an experience of playing TW games. Such people ought to know that one of the most important factors in battles is morale. So the crucial issue about these recruits is how high their morale is likely to be.
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