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, in addition to any statutory approval required (CE22.05). Trials competitors may use any appropriate helmet.
Helmets which also have a Snell M2005 marking are still valid provided they have the appropriate ACU sticker; this has been the source of some discussion in Motocross social media.
Further information and guidance is available in the ACU handbook (also online www.acu.org.uk/RidersMembers/ACU-Handbook.aspx