The minimum requirement for a UK rider to take part in an ACU event is for the helmet to have a Silver or Gold ACU sticker for off-road events and a Gold ACU Sticker for events on tarmac or other hard surface. Helmets are independently tested prior to receiving the appropriate sticker approval.
(https://www.acu.org.uk/news/2015/06/acu-helmet-stickers/)
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Shox ACU Approved Helmets
More about Shox ACU Approved Helmets
The minimum requirement for a UK rider to take part in an ACU event is for the helmet to have a Silver or Gold ACU sticker for off-road events and a Gold ACU Sticker for events on tarmac or other hard surface. Helmets are independently tested prior to receiving the appropriate sticker approval.
(https://www.acu.org.uk/news/2015/06/acu-helmet-stickers/)