Sorry if you only want British people to answer (Iīm American ) but I like your questions ...
1. Anything is possible (but learning a new language and understanding it when spoken by natives is rarely "easy").
2. I would say native speakers understand most (99%+) of what is said. But most groups have their own slang and this can prevent outsiders from understanding some of their speech. For example, many American adults complain that they canīt understand their own teenage children because of all the new slang used by the teens. And many work places use special words and phrases that people outside of the work place wonīt know. Also, I think misunderstanding amongst even native speakers occurs in all cultures and languages.
3. There are some dialects in America (that spoken by "hillbillies" that live in North Carolina, the ebonics spoken by some inner city African Americans, etc.) that I canīt understand completely. Also...when I was in England (especially trying to get to Abbey Road studios) I had a very difficult time understanding a lot of what people were saying. It was more difficult understanding English (my native language) when I was in England and Scotland than it was to understand Spanish (my second language) when I was in Spain!
4. On the bus trying to get to Abbey Road studios in England.
So, perhaps no native English speaker (unless incredibly traveled) will understand EVERYTHING said by every other English speaker.
Interested in Language