'I'm' (musisz uzyc czas przeszly bo to juz bylo)  lucky because when I was there it was The Summer 'Olympic Games' (mozesz napisac Olympics) at the same time. I had 'a' THE possibility to see (brak przedimka, choc wczesniej o tym pamietales) Olympic Games 'on live' (kalka z polskiego 'na zywo' - tak nie piszemy cos jest 'live' albo nie, tutaj wystarczy 'live'. London was 'the host' (mozesz daj,i chyba lepiej bedzie 'hosting') 'of' (niepotr) that event. 
 
I could see 'a' (cos tu nie tak, dajesz l. mnoga pozniej) professional 'sportsmans' (nie, l. mn to sportsmEn). The Summer Olympic 'programm' (aha nie wiedziales, to 'programme) featured 26 sports and 39 disciplines. In London'S Olympic Games 'took part around 10,800 athletes from around the world' (napisz to inaczej...around 10,800 athletes from....took part). The 'Olympiad' (Olympics) 'was' (were) very important for a lot of people.  
 
'Besides that' (nie, tu musisz inaczej, bo wczesniej mowisz o 'opening and closing' ceremonies, ale nie wiemy, czy chcesz powiedziec, ze je tez widziales. Jak tak, to musisz cos tam zmienic) I could see a 'lot of' (daj troche formalniejsze slowo) fans who supported their athletes. I think that the host country was (przedimek) focus of attention of the whole world for the period covering the event. (ja bym to zmienila...It gave people a chance to 'could' (niepotr) learn about the culture and history of London. 
 
It goes without saying that Olympic Games brought a lot of benefits for the London and the whole OF Great Britain. I'm 'confident' (cos mi to slowo tutaj jakos nie lezy) that I'll never forget THe participants, spectators and THE fantastic 'air' (air to 'powietrze' a ty masz na mysli 'atmosphere') in London.