Well, this is too much white text for no real plot spoilers, so spoiler warning but...

An upcoming ep of Agents of SHIELD is going to be connected to the movie so that might answer your question. But you know in the comics SHIELD doesn't show up all the time. It's always a problem with a shared universe why all the heroes aren't always getting in each others way as they try to save the world.

I haven't paid particular attention to this point, but it does seem that Movieverse SHIELD is mostly focused on protecting America and American interests in line with Ultimate Marvel's SHIELD. The TV show has been popping around the world but mostly to get weapons for the US or make sure they aren't in "enemy" hands. It makes them the good guys if you're American and a bit scary if not. From the trailer it looks like the new Capt America film will explore this view of the agency as not actually being the good guys a bit more.

So with an American-centric view, some weird stuff in London probably has their attention but would not be a priority. They'd probably let UNIT handle it since they deal with most of the alien stuff that goes on in Britain. Or as it called in the Marvel Universe: WHO (Weird Happenings Organisation - the similarity to a word associated with a certain Doctor is intentional). Or possibly the more supernaturally-focused MI-13 branch.

I also got the impression that the convergence was going to affect the whole planet, it was just centred at Greenwich - which only Jane and her team knew because of their specialised research - so plenty of other things for SHIELD to look at - I imagine Dinobot Island-type shenanigans going on and keeping the other Avengers handy.

And when things escalated, and an alien ship appeared, SHIELD probably set the helicarrier in that direction but it takes a while to fly from US airspace - by which time Thor had dealt with it all and the TV show will deal with the fall out.

Including maybe the very, very last end credits scene if budget allows? Did people stuck around for it? No one in my cinema except for me did, all bailing after the mid-credits Guardians of the Galaxy scene. Rule with Marvel Studios movies is always stay to the very end.