I'm inclined to agree, GG13.
We are still assuming an awful lot. On R4 this morning a retired Secret Services bloke said, well we cant assume its organised top down at all, it could be a rogue cell - after all, that has actually been the pattern in more recent years.
Lots of work will have to be done examining social media contacts of those arrested and so on. Goodness knows if Russia has a finger in the pie since it supports Iran.
As regards the EU and Brexit, I think our government is actually doing a lot to reduce numbers, much more than previous governments who just wanted to "ship off"the problem, which would have been impossible given the numbers now, and I doubt any kind of deterrent as many will just try again:
but we have lost the useful tradespeople coming over from Poland etc and lets face it many of them were very good.
Also it's easier to combat movements across countries when you are "in" the EU. However its very complex in Europe : Sweden used to be very very strict, but has just got a more left wing government and thats partly because its was felt the government was over punitive, as is the case in Spain, which isn the EU. and in Germany the rise of the right. (alongside, I might add, the rise of a strong left wing, Germany generally having very well organised folks...and also in Germany, its only in the old communist areas to the East the rise of the right has been so marked (more unemployment, underdevelopment ).
Very, very complex indeed.