London is libertarian, or old school liberal, not left-wing, and Boris Johnson did just fine as an old school liberal mayor before he started agitating for Brexit.

At least amongst some libertarians I know in the US, Brexit is definitely favorable to them. Is it a different brand of libertarianism?

It's a branch of libertarianism that believes that if laws and regulations must exist, they should be implemented at the highest possible level in order to ensure consistency and insulation from provincial arbitrariness.

The sort of 'libertarianism' that would advocate a world wide state than? To me that looks more like the ultimate in repression than in anything you might call liberty.

Exactly that sort of libertarianism. According to such a view, a world wide state would be less oppressive than a nation state because the center wouldn't have the sort of local knowledge to make repression truly effective, it wouldn't have the petty and internecine struggles that make local politics nasty, and it would have to fight through all the intermediate layers of administration in order to implement repressive policies. Because of this reason, small political units such as North Korea and Eritrea can be more oppressive than large ones such as Russia or China could ever hope to be, and most oppression in the United States takes place at the local rather than national level (e.g. excessive incarceration, civil forfeiture).

TL;DR: neighborhood bullies are far worse than the average dictator.

So from their perspective, the amount of repression one gets in a Soviet Russia or China is acceptable, and scaling it up to be worldwide could be considered libertarian? Or do they believe there is some way of stopping the worst people rising to the top in a worldwide government that doesn't exist at the Russia/China scale?

From their perspective, a cosmopolitan USSR or China would be more libertarian than a national USSR or China, if only because their leaders, no matter how authoritarian they might be, would have a harder time utilizing the local-level knowledge that makes repression effective. They would have to deal with a wider variety of interests whose support they would need to stay in power, which further dilutes their power. In addition, citizens would have more options to play different levels of government against each other; if the state-level government is becoming too oppressive, one can always appear to the supranational government, as Europeans do to the ECHR or ICJ, or as southerners did to the Eisenhower administration in the 1950s.

In short, no dictator can oppress me like my family and neighbors can, which is why any dictatorship with a drop of wisdom would try to co-opt them.

I fail to see how a worldwide China has any less local knowledge than a China sized one - it is not even a factor of 10 difference in population. Also, a cursory look at history would suggest that dictators have a lot more power to oppress you than your family and neighbours. I assume you are misrepresenting the ideology due to lack of space - are there any books/tracts/thinkers you can point me at with more detailed descriptions of it? Thanks.

Too many people conflate what "globalism" (of the E.U. variety) has become with the libertarian program of open borders and free trade. The E.U. is a long march towards political unification. That's why you have libertarians who (rightly) oppose it.