Yes, it has to be the RNLI, the most prominent and populous institution dedicated to saving lives at sea. All other efforts would be small and very local, and thus not good enough to quote as an example. Calling it "English Association" would have come from the the the habits of Kropotkin's mother tongue (Russian), the conventions of the original language the book was written in (French) and the need for easier comprehension when talking about a foreign organisation. After all, the aim of the book was not to offer precise information about RNLI but rather to use RNLI's workings as an example in support of a cause. Being an "anarcho-communist", Kropotkin seems to have deducted the principles on which RNLI functions and worded the resulting interpretation in a way that is suitable for his book.

The usage of the terms "free agreement" and "anarchic" needs to be viewed through the lens of the anarcho-communistic philosophy, rather than as their direct dictionary definitions. In "The Conquest of Bread" Kropotkin puts forward the ideas of a decentralized economic system based on mutual aid and voluntary cooperation. And that's exactly what "free agreement" and that "anarchic organisation" are about. The lifeboat system in Britain is not a state organisation. It is based on volunteering and public support, i.e. it is decentralized and relies on mutual aid - a perfect example for a working system of the kind that Kropotkin needs to prove possible and successful.

The RNLI proves that Kropotkin's ideals are not just imaginary and theoretical but they are put to practice somewhere in the world. Their benefits can now be argued even in front of somebody who is not philosophically inclined or versed in complex thinking. A real, working example can be shown for that person to see for themselves. All Kropotkin needs to do afterwords is to assign to it terminology taken from his philosophy.

Part of the economic theory of the anarchic communism is the abolition of wage labor - no wages in the RNLI, they are all volunteers, working for the common good not for money. There are self-determined needs (the obvious presence of a boat in distress), there are no orders coming from the centralized state, and the participants are free to engage in whatever activities they found most fulfilling - with RNLI that's the saving of human lives. It's a bit more complex now, with rules about health and safety etc but in the olden days things were much simpler - you see somebody in the water, you pull them out. It's been a long-standing tradition along the coast, before the RNLI, with esteblished lifeboat crews in each fishing town (they did join later on as there were funding benefits). Technically, the RNLI didn't add any principles to what was already been done as a tradition (its prime purpose was getting funding). It's part of the British way of life.

Those people are there to help, of their own volition, based on their own values and beliefs, without expecting remuneration. There are no arbitrary pressures inducing people to carry out anything - an anarcho-communist's dream. There is no numeric economic value assigned to the saving of a life and there are no office hours - according to Kropotkin, "masuring [the work] by hours spent on the work also leads us to absurdity". Everything about RNLI just fits with the theories of the anarchic communism. The RNLI members are of the same rank, there is no administration, no distinction to be drawn between the work of each man - they jump in their lifeboats and go out saving whoever's in the sea, whenever it happens. There is no lord owning the boat or the tools of trade, which is great as the anarchic communist reject the concept of ownership in favor of the concept of usage. RNLI ticks all the boxes - about motivation, individualism, freedom and work, about property and the commune as an economic democracy, about free association as opposed to state control. You can pull a good example from RNLI about each one of those essential anarcho-comminist points. The RNLI may not be a nation or a society, but it's about the only working example out there. It proves that people can do this for real and what's more, they are not ancient tribes or underdeveloped islands - it's all happening right here, now and in the very heart of capitalism. Bulls-eye! Communism will ensure well-being for everyone - just like the RNLI who makes sure everybody is okay at sea, not just the big boats of the rich and powerful. The essentials are produced for all people, they are distributed properly and nobody would have unmet needs. The "product" of RNLI's work does not belong to any individual but rather to the commune as a whole - saved lives bring respect to the organisation. RNLI has egalitarian principles - all people are equal, whoever you are, you don them rubber boots like everybody else in the boat. There are no dictates of religion or morality, it's free association. The workers own the workplace, the RNLI is a cooperative, a commune. According to anarcho-communism, the future society will be organised territorially through free localities - just like the RNLI where each town has their own lifeboat. It's work but it's the community doing it themselves, each one autonomous yet part of a wider communal-confederation, linked together by voluntary contractual agreements. The workplace is not a site of struggle against the establishment. And each workplace is in free association with the others, no centralism. That's Kropotkin's anarchy right there. In "The Conquest of Bread" there are also a few chapters on "Objections" where he mentions what may be the prominent objections to his theory. And the existance of RNLI can be used as a response to those: some people may be lazy and may not work willingly but the RNLI members are willing, so it can be done when people have the necessary free time and enjoy their job. It's not just a theory, such people are real, doing their thing for the collective.

If you are in Britain, do visit one of the lifeboat museums around the coast. As for books, perhaps the story of RNLI's founder will provide more about the "principles" behind it: "The Lifeboat Baronet: Launching the RNLI" by J.Gleeson. The RNLI itself is not very philosophical or theoretical at all, it's a hands-on experience with no waffle. Books on RNLI tend to be about courage, good judgement and determination rather than "anarchy" and "association". There is a lot of volunteering and community work in Britain and RNLI is just part of that. I'm afraid Kropotkin has just put a fancy name to it. If you are interested in the reality of the lifeboat crew, there's a little book on one of the pioneers (1917 onwards) "Henry Blogg of Cromer: The Greatest of the Lifeboatmen" by C.Jolly.