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Trust me on this, as long as it has pedals that could propel it and brakes on all wheels that can stop it. It can pass the 250w LMP inspection.
(If you want a long pretty boring read https://assets.publishing.service.gov.uk/media/6308ab748fa8f55361ddd828/motorcycle-single-vehicle-approval-inspection-manual.pdf or a slightly less boring one https://www.lfgss.com/comments/17076823/ Spoiler he did it)
They are already legal. It’s a one time inspection that costs £50. No insurance, no tax, no license.