Anyone in UK can register a a supporter of Labour, and get to vote in the election being held for Labour party leader. If you're a member of a Trade UNion thta is afiliated with the party, and pay into it's political fund, you can register for free, otherwise you have to pay £3.

In either case you have to declare you support the values and principles of the party - which isnt the same as actually joining the party, which is fair enough since the Labour Party typically forgets its values and principles.

Jeremy Corbyn is an exception to that, which is what makes him a different kind of candidate, and one a lot of people are rallying to. Meanwhile the party establishment are horrified.

Perhaps the most appealing thing about him is that clearly he has no personal ambition to high office whatsoever. He's running because he thinks it,s his duty. That's pretty unusual among politicians in any country.