This is a question for those who know the layout of this venue/station interchange well.
1) Do you see this as an attack on the UK rail network/premises directly?
2) Was the place the device detonated on railway premises or the venues?
3) Is the layout/people flow between the venue and station/transport links similar to say the Greenwich/02 and North Greenwich transport interchange- with the vast majority using this one route to leave?
No to 1 and 2 I think, don't know about 3.
The Arena is built on the north side of Victoria station, and partly above its northern platforms.
It's actually built on land freed up by the closure of some of the old through platforms.
The city-side pedestrian access to the Arena utilises the station bridge to reach the foyer on the same level.
The foyer of the Arena is beyond the station boundary, so the attack doesn't seem to be directed at the railway in any way.
However, it's possible some of the station structure has been damaged by the explosion.
I don't know enough about the Arena to say if the station access is the main/only entrance.
I think there's also a Arena exit at the far west end of Victoria platforms.
I'm sure the locals on here will know the detail.