I applied via London - they didn’t ask for any additional documents - but they did ask for flight ticket as you say. Personally I wouldn’t take the risk of applying via UK but from another country
there is lots of conflicting information on Google. Better to also ask people that have done it. You know the sort of people you mind find in an advice group 🙄
no one can answer this for you - it depends on the source of the money, whether you have already paid tax on it (or earned it in a year you were not tax resident if you are a new tax resident), whether your home country has a DTA, what exemptions you can claim. It is complex but there is enough content around (including youtube) that will help you get a feel/estimate.