The AutoCycle Union (ACU) Gold Sticker has long been a benchmark against which the quality of motorcycle helmets can be measured. Moreover, many motorcycle racing organisations will not allow riders to compete if they do not wear a helmet which displays the ACU Gold Sticker and meets the standards inherent to it.
There used to be both Gold and Silver stickers, where the Gold stickers met the BSI Type A standards and the Silver stickers met the BSI Type B. The Silver stickers are no longer in use though, as the introduction of the Economic Communities of Europe (ECE) standard 22.05 provided a clearer, more stringent testing system which made the Silver standards redundant.
In the same way, the ACU Gold sticker is also undergoing a revamp, where helmets sold in the U.K. will be granted a rating between 1 to 5 depending on how the helmet performs under testing. This is not yet in place though, with the testing criteria currently being decided.
For the time being though, it is safe to assume that a new helmet which qualifies for an ACU Gold sticker is a high quality lid which has met and sometimes surpassed tough test standards.