Well done antgel, good to hear you're started.
As you say, the route you take depends on your priorities. Enjoy the
ride, don't worry about going a little further. Learning to ride with
traffic will take time, I'm happy to cycle down the Euston Road, but
only if I'm in a hurry to catch a train!
Often, designated or signed cycle routes are not very good, sometimes
they're even dangerous. Over time you'll get to recognise where they
are useful and avoid the other ones. In London there's a lot of scope
for trying out different routes, so you definitely don't need to go on
the "main routes" - if by that you mean the ones signposted for the
cars. As you get to know a route, try out different side or parallel
routes to work out the best one for you - you can often make better
progress (and breathe less fumes) than on the main road.