A couple things about commuting, even on main roads....
Try all of the routes. Try them different days, or ride each for a week at a time. Get to know them well. It will provide you with variety in what may otherwise become repetitive. It will also give you alternatives in case of closures or problems.
I think what may seem intimidating now, will seem less so as you gain confidence. Agressive driving does occur, and more frequently in London than other places. However, it's not so common as all those videos on YouTube would have you think.
That said, there is quite a bit that an individual cyclist can do to prevent and avoid bad experiences on the road. Get a book called Cyclecraft
by John Franklin http://www.cyclecraft.co.uk/
As for bikes... have a look at what's available at http://www.stationbicycles.co.uk/
They carry practical second hand bikes that have been serviced and checked over.
A full sized bike will probably be more comfortable and faster, though I know people ride folding bikes for long distances. A folding bike does have the advantage that it will go on the train.
If it were me, I would make a list of purchase criteria, then go ride some bikes. Probably at more than one shop. I think that LCC have a list bike shops on their site.
Some criteria for purchase that you may want to include are:
*size - it's got to be the right size for you or it won't be comfortable to commute on; a fitting is best, or help from a knowledgeable professional
*takes mudguards - this is important for bad weather riding; otherwise you'll end up with slurry of road gunge up your back
*luggage carrying capacity - will you need to carry books? laptop or tablet? clothes? You will probably want a rack if you need to regularly carry more than just puncture repair equipment
*takes winter tyres - if you plan to commute year round, you may want to put winter tyres on the bike; if so, it will need extra clearance for those tyres
*gearing - this might not be so important if you have a flat commute and only light luggage, but if you also want to do the shopping on it, or have some hills to consider (I don't know the route), you may want low gears.
Other than that, you may have your own criteria, such as type of handlebars,
“In some ways, it is easier to be a dissident, for then one is without responsibility.”
― Nelson Mandela, Long Walk to Freedom