The Irish folk singers use the word 'limey' to refer to the British. This is a predominantly American term which the Irish never use.
Demolishing a building in any city requires a permit. It is not possible to complete all the legal requirements and obtain such a permit in only one day, and by the time Gary would have done that, he would have long figured out Harry was a con artist.
It's unlikely that "Harry the Hat" would be unknown to Gary. Since he's a con man, it would stand to reason that the would not limit his victims to the patrons and employee's of Cheers. Harry would probably have targeted all the major bars in Boston and they would know about him and his record.