Simile:

“I’ve been locked up as much as a silver tea-kittle”

Magwitch was talking about how he had, been through everything in prison. So by saying he has been locked up more than silver tea-kittle the connotation is that he has been locked up in prison so much. This implies that he was being kept secure and that he couldn’t get out because, the owner would keep the silver tea kettle secure because it is valuable.