This is so not true! If you seriously think that Nagini and the snake set free in the zoo are the same then your delusional. Let's examine the evidence: 1. The snake Harry set free was a Brazilian Boa Constrictor, and its sex is ambiguous (Harry Potter and the Sorcerer's Stone pgs 27-28 US Paperback 1st edition). 2. The average size of a mature female boa is between 7–10 ft, whilst it is 6–8 ft for the males. It is common for female individuals to exceed 10 ft, particularly in captivity, where lengths of up to 12 ft or even 14 ft can be seen (http://en.wikipedia.org/wiki/Boa_constrictor#Size_and_weight). 3. Nagini is at least 12 feet long (Harry Potter and the Goblet of fire pg 12 US Hardcover 1st edition). 4. Boa constrictors are not venomous, though if left unattended their bites can become infected (http://en.wikipedia.org/wiki/Boa_constrictor#Behavior). 5. Nagini has to be 'milked' for venom. She is a venomous snake (Harry Potter and the Goblet of fire pg 7; 656 US Hardcover 1st edition). 6. Boa Constrictors have arrow-shaped heads (http://en.wikipedia.org/wiki/Boa_constrictor#Coloring). 7. Nagini has a triangular shaped head (Harry Potter and the Goblet of fire pg. 12 US Hardcover 1st edition). 8. Nagini is said to have a diamond patterned tail (Harry Potter and the Goblet of fire pg. 13 US Hardcover 1st edition). 9. As seen in the linked picture, Brazilian boa Constrictors have splotchy round patterns (http://www.generalexotics.com/images/Brazilian_Rainbow_Boa.jpg). 10. The Boa Constrictor was in a muggle zoo and could not free itself, meaning that it has no magical powers, as evidenced by the fact that it was Harry who freed the snake (Harry Potter and the Sorcerer's Stone pg 28 US Paperback 1st edition). 11. Nagini is a magical snake, if only because Voldemort has placed spells on her, though it is quite possible that she was magical before this. Also, her venom is magical, preventing wounds from closing, so she MUST be magical. (Harry Potter and the Order of the Phoenix pg 488 US Hardcover 1st Edition).
So given this evidence, we can conclude that given size alone it is possible that the snake could have been Nagini, given that the snake could've been female (50% chance) and since it was bred in captivity, reaching the required length. HOWEVER it is unlikely that without being a magical snake it could have escaped the zoo and survived in the real world of England for very long, especially traveling around 200 miles. I say 200 miles because the zoo is in or around little Whinging correct? And Voldemort is hiding out at his father's old home in Little Hangleton. Now in Chapter One of Harry Potter and the Goblet of Fire (Aptly titled 'The Riddle House') a lot of stuff happens. Harry witnesses it all in a dream. I can tell it was 200 miles because the last line of the chapter is "Two hundred miles away, the boy called Harry Potter woke with a start." (Harry Potter and the Goblet of fire pg 15 US Hardcover 1st edition). So IF the snake was even a girl, and managed to travel the 200 miles, it COULD be Nagini. HOWEVER this is all circumstantial. Now another thing is that it clearly states that Nagini has a triangular head, as do Boa Constrictors, because their heads are shaped like arrows, which are triangles. HOWEVER several other kinds of snakes including pythons (http://www.pembinavalleypets.com/wp-content/uploads/2012/02/python03.jpg), vipers (http://snakespictures.net/Viper-Snake.jpg), anacondas (http://www.stormchaser.ca/wildlife/venezuela_wildlife/Anaconda_06.JPG), and Black Mambas (http://www.animalwallpapers.us/images/wmwallpapers/Black-mamba-snake-1.jpeg) have triangular heads. It's sort of a snake thing, to have a triangular head. Rather snakish really. moving swiftly along, Nagini has a diamond patterned tail. Brazilian Boa Constrictors do not, as can be seen in the picture linked above. (Also linked here for convenience: http://www.generalexotics.com/images/Brazilian_Rainbow_Boa.jpg). Finally the most pressing point, the one that can't be argued and downright PROVES that they are not the same snake. THE fact that could have put a simple stop to all this nonsense if people actually used their brains: Boa Constrictors are NOT venomous and Nagini is CLEARLY A VENOMOUS SNAKE! Voldemort uses her (magical) venom mixed with unicorn blood to keeping himself alive in his twisted baby form (Harry Potter and the Goblet of fire pg. 656 US Hardcover 1st edition). Furthermore, we can tell that it is magical venom because it clearly states on page 488 of the US Hardcover 1st Edition of Harry Potter and the Order of the Phoenix that Arthur Weasley cannot be released from the St. Mungos because there is poison in the snakes fangs that keep wounds open. If that's not a MAGICAL, VENOMOUS snake, of which the Boa Constrictor certainly was NOT then I'm not sure what is.
Now that's all I've got. If that doesn't convince you that they're not the same snake then you're a hopeless case. I have spent plenty of time researching this and I am firm in my belief that this is the case. Unless you find me actual PROOF from an actual VALID interview with J.K. Rowling where she EXPLICITLY STATES that they are, in fact, the SAME snake then I will not believe you. Even then, I will still point out that J.K. Rowling has a continuity error because BOA CONSTRICTORS ARE NOT VENOMOUS!!!!!!!