SITUATION A
If I convert BTC to USD in my Gemini exchange, will that already be subjected to tax? Will USA tax me considering I do not reside there? Now of course
Are you a USA citizen? (I assume you aren't, because of the context of your questions). If you're not a USA citizen the USA will not expect you to pay taxes on investment gains. You don't live in the USA, you don't work in the USA, the USA has no claim to assess tax on your income.
Basically, I want to know the best way to lessen the amount of tax that I have to pay.
The best option is to repatriate you cryptocurrency into the country and account that will impose the lowest tax rate on your investment gains.
You need to understand first, will the UK assess taxes on investment gains if you live and work in the UK (but are not a citizen)? If so, you'd likely owe the tax to the UK regardless of where you repatriate the money.
If you repatriate the money to the Philippines, will that exchange/company record the transaction and report it to the UK?
Do I have to record every trade transactions that I make?
Yes, from crypto to fiat. Each transaction represents a taxable event.