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thanks, broDo you want to make your battery cables longer?
Re how you are in London, is this for a boat on the Thames?
If so, I understand that inland waterways boats have to conform to the Boat Safety Scheme - here is a link to what they say about electrical safety (there are other links as well on the site) -
The Electrical System | Boat Safety Scheme | Go Boating - Stay Safe https://www.boatsafetyscheme.org/stay-safe-advice/electrical-safety/the-electrical-system/