A few things to round up.
Although St Thomas’s Hospital is on the opposite bank of the Thames, the most seriously injured have been transfered to the specialist Trauma Unit at King’s College Hospital which has a rooftop helipad. King’s received 8, two are critically injured and 6 stable.
If you are worried about friends or family, try texting them. You may not get through by voice or a quick reply as they could be travelling and unable to get reception. Your text will get through when, for example, they leave an Underground station.
Parallels have been made with Berlin and Nice. A closer comparison is the attack on Lee Rigby where a car attack was followed by the use of knives. The other attacks could well have been inspired by the killing of 6 people and injuring of 15 in a crash in Glasgow in 2014.
The point on Westminster Bridge where the car first left the roadway is where there is a souvenir stall with large Union Flag umbrellas and other similar goods on clear display and visible from the road.
The rear number plates of the car used were clear so it can be readily identified from the many CCTV traffic cameras around London. Westminster is well within the Congestion Zone which has number plate recognition cameras at its entry points, to facilitate charging. That should make tracing the last part of its journey fairly quick.