That is quite easy to answer. The UGLE is the United Grand Lodge of England, the one GL in England, that is recognized by all the other regular GLs in the world, and the other is not. You may find the word "regular" in every two sentences on the RGLE website, and they also may list a number of other "regular" websites of regular Masonic High Councils, but there is nothing regular behind it.
For example, as it concerns where I am from, they list a Masonic High Council of Germany. That body is not even mentioned by the irregular Grand Lodges here, and it is completely unknown to any regular mason I know. It lists no name, the "list of regular lodges" is empty. There seem to be some lodges behind that organization, like in Italy, but only a few.
It is like the Grand Orient of the USA.