We've taken Norwegian and really enjoyed it - they also fly to many destinations, not just Scandinavia. They fly out of Gatwick and also have specials to the US.
For learning the tubes and busses of London - download and use the Citymapper app, or utilize it online. It lays out all the bus and price options across multiple types of transport - bus, taxi + bus, walking, etc. Eventually you will realize that there are certain areas where it is actually easier to walk to the next stop on the Tube than take the train.
