First of all I would make certain that both my cat and I were indoors. Then I would take photographs of the reptile. This would be in order to sell them to the newspapers, as a picture of an escaped crocodile or alligator in England would be newsworthy and valuable. Then I would call the police and the Royal Society for The Prevention of Cruelty to Animals to come and take it away.
If it happened in my previous residence, I would do nothing except call my then landlord to come over, in the hopes that the carnivorous beastie would eat him , feet first and very slowly.
If Iived in a country where such reptiles were common, I would wait indoors until it went away. I would also call the emergency services if possible.
If you are in such a situation and you want some fun, I can call my previous landlord and tell him that there is a penny to be picked up in your garden. The mean old rat is bound to go flying over there to pick it up. Then you can watch the ferocious reptile eat him. If you have a camera, you can send me copies of the photos. Much appreciated.