There are two types of constituency: borough and county. In a borough constituency, the mayor or chairman of the district is the Returning Officer. In a county constituency, the Sheriff of the county is the Returning Officer.
Campaign spending limits are different depending on whether you are standing in a borough (burgh in Scotland) or county constituency. This is covered in detail in How much can you spend?
Typically, borough (burgh in Scotland) constituencies are mainly urban while county constituencies are mainly rural.
If you do not know which type of constituency you are standing in, you can obtain this information from the (Acting) Returning Officer.