Advice on London itinerary + some questions

Hey, it's my first time travelling solo and I'm slightly nervous. I didn't do very detailed planning yet because of the exams and I'm leaving right after them.

Dates Length: 6 days.

22 June, landing in the evening. So not much to do first day. leaving 27 th very early in the morning. So I have pretty much 4 days to do actual sightseeing.

Budget Hoping to spend not more than 100~200 euros. I have brought 3-day London-pass and 20 GBP oyster card.

Want to avoid:

overcrowded/touristy places

Need help with

• Indian food
• Fish&chips
• Tea

Generalised plan

Day 1:

Since I arrive in the evening (~16:00) I won't have much time to do a lot, so I don't want to activate my London Pass yet. Probably just walk around and find some food.

Day 2:

Start my London Pass. Hop on-off bus tour and try to visit as many as possible attractions it passes through (Buckingham Palace, Tower of London, Tower Bridge, Trafalgar Square, Big Ben, River Thames, Westminster Abbey). I think that's enough for single day? + get food in between.

Churchill war rooms

Day 3:

HMS Belfast, Churchill war rooms, zoo Tower of London with food in between and maybe thame river cruise at the end?

Day 4:

Not sure, since I'm kind of running of big places inside London-Pass to visit. Maybe national gallery, Tate Modern? Zoo

Day 5:

No plans, just walk around and get lost + Hyde park + National gallery + British museum

Day 6: Leaving early in the morning.

Some questions:

• Can I use oyster card to pay for rail from Gatwick airport to London?
• Can I top up oyster somewhere in the Gatwick airport? Got 20GBP, most of it will be gone just to get from Gatwick to London.
• How easy it is to find street food in London? I prefer eating while walking than awkwardly sitting by myself in a restaurant.
• What am I missing?

Thanks