The basic answer to this is that he will be sentenced to death if he is found and identified in London or anywhere else in Britain.
The reason for this is found in his original sentence. He was sentenced to be "transported." This meant that he was sent to Australia instead of being executed or imprisoned in Britain. But the other side of that deal is that he was never allowed to return to Britain.
Now that he has returned, he is risking death. You will find out later in the book whether he will get caught and face death or not.