Well my strong advice will be do not underestimate the amount of time you will need to park and get into the school. The streets will be heaving and you need to plan somewhere you can park which should be some distance from the school. Police will be out ready to ticket.
That's very true - best to arrive early. We did SET in Wilson's last year, the afternoon slot, and the congestion was unbelievable - not only in the immediate vicinity of the school. There were a few angry residents who were unable to leave their driveways as they were blocked in by parents dropping off for the SET. Bear in mind that there are a couple (I think) of primary schools close to Wilson's and their usual drop-off/pick-up traffic is still there. Allow plenty of time to get there and assume it may take you some time before you are able to drive off if you go by car.
On a different note - the forecast for Wednesday is another hot day, so please make sure DCs have enough water with them. Good luck to everyone sitting the SET.
Agree with Purple. Dropped my DD for the morning session and it does feel that using public transport is the best option if you can, although given the temp, its best to take adequate care as well and lots of water.
All the best to everyone who are giving SET today