I live nearby and walk past where the attack happened everyday, so I can give you a well informed run down.
The Finsbury Park mosque is in St. Thomas Road; yes it is the same mosque that was quite controversial in the past, but that changed quite some time ago. It has already been the subject of attacks by anti-islamic types, you may remember it was actually national news when somebody tried to firebomb it. After that happened, they kept a police officer on duty outside, but I think that's since stopped.
The Muslim Welfare House is on Seven Sisters Road, around the corner from the Mosque, roughly 3-4 minutes walk away. It's right next to one of the entrances to Finsbury Park station and tends to be quite busy with a pretty narrow pavement given the amount of foot traffic. This is just before the railway bridge, so when you see the live news coverage on TV, they are reporting from the other side of the bridge, looking in that direction.
Where the attack actually happened is on a road called Whadcoat Street off of Seven Sisters Road, it's pretty much a tiny cul-de-sac with only room for three or four cars. This is still a minute or so walk from the Muslim Welfare House, as you have to cross a busy road. Over the past six months or so, I've noticed that a group of (mostly young) Muslim men gather in the early evening in this road, presumably after prayer. They are usually playing some kind of game on the street, I don't know what it's called though.
The attack happened at that particular spot, with witnesses describing how he left the bus lane and turned left into the road.... it's a very sharp turn, so I'm not sure it's something you could tackle at a particularly high speed, otherwise the van would likely be in the wall.
The attack actually happened closer to a Church than it did any Islam mosque or centre.