By law, the UK only insists on wearing a crash helmet whilst riding on the road however it is strongly recommended that you wear one at all times to help protect against any head injuries from accidents or flying debris.
All helmets should be manufactured and tested in accordance with the current relevant EN/BS standard. They should also have a chin strap and be able to be used with eye protection, either in the form of a visor or goggles. Your helmet should be chosen based on the way in which the quad bike will be used. The harder the surface and higher the speed the greater the degree of protection needed.
It is important to note that forestry helmets and industrial hard hats are not acceptable for any UTV / ATV operations.
You should also wear clothing that is strong and covers the arms and legs to help protect against any cuts and scrapes. Additionally, wearing gloves adds more protection as well as adding greater grip on the handlebars for a safer ride. Whilst riding you should also wear the correct footwear, this included boots that are strong, supportive and cover the ankle.