Since I was a bit worried about what to do regarding my travelling in London, I thought it best to make an update here!
I ended up ordering an 7 Day Travel Card online, which I think is a really good option, but I was really worried I wouldn't get it in time for my trip. I also had no opportunity to choose an insured delivery option, as I wouldn't be able to be at home, waiting to sign for the delivery ... it was a bit of a chance, but I felt I had to take it.
I ordered the travel card early early on Thursday morning, and during Thursday afternoon I got an email saying the travel card had just been sent, and if it didn't arrive within five working days I shouldn't contact them.
I was surprised, to say the least, to find an envelope from Boxoffice among the rest of my mail yesterday afternoon (Friday) - with my Travel Card!! :)
It feels SO good to actually have the card with me when I go, so I don't have to think about that the first thing I do when I arrive in London.
I do know, however, that I really should take the underground from Liverpool Street station when I arrive - it's a bit longer walk from Edgeware Road Underground station to my hotel than from the bus stop, but the trip takes like 20 minutes .... in comparison with the bus which will take an hour.
Don't think I'll decide on that right now either, it feels a bit adventurous to try on the underground system for the first time ever when I have just arrived in London, it's a bit late at night, and I need to check in to the hotel as soon as possible - but still, if it cuts my travel time in half, it's certainly worth thinking about ...