Having cycled 7-8miles from north london into holborn and reverse for the longest time. One of the best words of advice is to keep your head on a swivel and shoulder check.
Depending on the time of day you commute, you'll need to pay attention to pedestrians as well as some of them have the habit of stepping out in front of oncoming traffic without looking or running through the gaps between vehicles while at traffic lights. Other cyclists can occasionally pose another risk as their heads are glued on to face one direction and dont shoulder check AT ALL or some of them might be too close for you to pull out and you end up making contact with them and knocking them off their bike.
While most drivers are generally polite and let you be on your way. Ive had quite a few of them try to run me off the road or not give way leaving me in a rather troublesome spot of slamming on my brakes to avoid crashing into the vehicle in front rather than give way and let me pull out.
But i share other peoples opinions. Ride the route on your days off to familiarise yourself with the route and the general environment.
Be particularly weary of big vehicles like HGVs, Busses/coaches, tipper lorrys, vehicles loaded up with construction materials at traffic lights as you never know if they will turn into your path and its best to stay behind them at traffic lights. Dont feel that you have to squeeze through the smallest gap just to get to the front of the queue, you might get lucky doing it once or twice but it is really dangerous and wont end well for you.
If a really long vehicle passes you. Slow down just in case the nutter tries to squeeze you to the kerb - This happens to me often and its even more important that you do this if there is a left turn coming up.
Be alert.