that为主语，has changed谓语为单数，很容易排除复数先行词的。

从句意上that 只能修饰new rules and guidelines。至于has单数可能笔误。如果想修饰整个事件，要用逗号隔开分局，引导词改为which 才说得通。

In 1990, new rules and guidelines were adopted in the United Kingdom that has changed the whole field of Archaeology in that country.

网友很可能抄错了句子。漏掉了一个关键连词 and！

In 1990, new rules and guidelines were adopted in the United Kingdom and that has changed the whole field of Archaeology in that country.

加上 and 后成为并列句。that 变成指示代词，指代前文整个分句。