You can't stop geography, cultural background and social issues.
If the US was geographically closer to Europe, and had the same cultural history as Britain, it would be seeing plenty of home grown extremists itself right now.
Of course, it is a problem, but seeing it as some sort of policy failure is exactly the simplistic approach that got US into so much trouble in Iraq and other countries. We are talking about situations that are the end result of a many years of cultural shifts, plus natural geography and proximity.
The reality is that, over time, unless things change, the odds of extremists being spread across many European countries is very likely - Britain is simply in a position due to its history where it's leading the way (not intentionally of course).
Personally, some serious thoughts on policy regarding what culture Britain actual wants and policies around that seem to be required. Right now Britain has changed hugely in a very short period of time, from a cultural perspective, and it's all been organic - i.e. unplanned in any way.
Given it's roots in India and Pakistan, social and economic inequality plus the fact integration of immigrants, particularly those with other religions other than the nominal 'official' religion of Britain - i.e. Christianity - has been pretty poor, with most living in tightly insular communities, plus the obvious lure of a 'British' extremist, it's of no surprise at all to me that we're seeing what we're seeing.
Another thing that would help would be if the US actually listened to Europe on the effects it actions will have on certain individuals living in Britain and other European countries - historic foreign policy by US has often handed certain movements exactly what they need to recruit from certain communities on a silver plate.
Try to be reasonable... its easier than you think...
