N reg and G reg, such a difference??

Ive just started my PPL aged 16 and i will have it at 17 obviously because i cant get it earlier, neway whats all the N reg and G reg, whats the difference, ones UK and one American, but whats all the fuss about?? and is it true that SEP aircraft cnt fly over central London, so when i do pass before i can go over london illl have to get multi engine rating and plane.

Can some experienced pilots tell me come more information on what i can do and where i can go with SEP, becuase when my dads gonna buy me a plane, i think a Piper Saratoga II TC, Commander 115 TC, Socata TB21 trinidad, Beech A36 or a Cirrus SR22.

Which ones the better aircraft?

Thanx

Oz