To get a UK passport you must be a citizen of the UK.
A passport is proof of identity and proof of citizenship. Therefore you can only get a passport for a country if you are a citizen of that country (with very few exceptions).
No, you cannot fly with an expired passport.
No, you cannot obtain your passport number before your passport is delivered to you.
You cannot travel on a cancelled passport, but you can carry it with you. You have to travel on your renewed passport.
No, you cannot fly with an expired passport. It is important to have a valid passport when traveling internationally.
No, you cannot obtain your passport number before receiving your physical passport.
No, you cannot enter the US without a passport.
No, you cannot leave the US with an expired passport.
No, you cannot leave the US without a passport.
No, you cannot leave the country without a passport.
No, you cannot reenter the US without a passport.
No, you cannot travel domestically with an expired passport.
No, you cannot travel out of the country without a passport.