blaire.witch
1. Google says there's a Catholic church in Marylebone, near Baker Street station. I vaguely remember seeing one around Covent Garden as well, but I can't remember the name, and the entrance was in a little street...Maiden Lane, I think it was.
2. Oyster -- you can always top-up (or reload) if it runs out, but I agree that £20 should be fine. It depends on how often you use the tube/buses, and especially since you're doing the Harry Potter studios, that's all the way in the edges of north London, so might cost you £4+ return.
3. My friend and I did the Complete Studio Tour Package. I'd take a photo of the Souvenir Guidebook but I'm at work and I can't remember where I hid it! Haha. It depends on how much a big fan you are, I suppose. There are no tour guides inside the studio, so the Digital Guide, erm, guided as along. I can't recall if you can just pay additional on the day for audio guides when you get there. There will be parts where you might want to shell out more money, like green screen photos of you in the flying Ford Anglia, riding on a broomstick wearing robes, and the butterbeer.
4. You're going in October, if I recall correctly? Buckingham Palace is only open during the summer, til the end of September. Another castle near London is Hampton Court Palace, which is around zone 6 of London, but you have to take the national rail for it. Anyhoo, it's not occupied by the royal family, but it's apparently been built by Henry VIII (the one with the six wives) to rival Versailles. They tried to preserve how it was during that period, but what I love most about it is the gardens.
Windsor Castle, on the other hand, is still a royal residence, and it's interiors are definitely much, much better than Hampton Court in terms of grandeur. No garden on this one, save for the Walk which stretches for so long I didn't even attempt to walk it because I walked around the city centre plus the castle, so...yeah.
5. The Tower of London museum is good. I'd recommend doing the full thing, but you can still take nice photos without paying.