I'm very curious about where the attackers got those firearms. France isn't as heavily disarmed as England, but it isn't exactly the United States for firearms availability; if any of the offenders' weapons can be recovered, I'm sure there will be interesting things to learn from their serial numbers.
There are open borders in most of EU and it's a piece of cake to buy automatic weapons in eastern Europe, gangs in Sweden import theirs from Slovakia for example.
I hadn't known that it was as bad as that! (Other posters here mention old Soviet arms caches, and the general surplus of weapons spread by the fall of Communism.) It sounds like the UK benefits quite a bit from being on an island... (Are they in Schengen or not?)